Why Do OVH and Scaleway Block Port 25 for Outbound Email?

You're trying to send a transactional email—password reset, order confirmation—through an OVH or Scaleway server, and it fails. No error message, just silence. You check your code, your SMTP settings, your credentials. Everything looks right. So why won’t it go through?

The issue isn’t your setup. It’s port 25. Both OVH and Scaleway block outbound port 25 by default because it’s been weaponized for decades. Spammers used it to send unsolicited messages at scale, often from hijacked servers. Now, cloud providers treat port 25 like a known threat vector—blocking it isn’t an accident, it’s a necessity.

Think of port 25 like a backdoor that once let anyone walk in. Today, providers lock it down to protect the entire network. If you're sending email from a server hosted on OVH or Scaleway, you need to understand why port 25 is blocked and how to work around it—without damaging sender reputation.

Key takeaways

  • OVH and Scaleway block outbound port 25 by default to stop spam abuse and protect their infrastructure.
  • Blocking port 25 is a standard practice across major cloud providers, including AWS, Google Cloud, and Azure.
  • Reliable email delivery from these providers requires using alternative ports like 587 (submission) with SMTP authentication and encryption.

How Do OVH and Scaleway Port 25 Rules Impact Email Delivery?

Outbound email from servers hosted on OVH or Scaleway will fail if you try to use port 25 directly. Both providers block port 25 by default to prevent spam abuse, which breaks any self-hosted email service, automated script, or application relying on direct SMTP over that port — even if your messages are legitimate transactional or marketing emails. Without alternative routing, your emails won’t leave the server.

Why Port 25 Is Blocked — And Why It Matters

OVH and Scaleway block port 25 on their cloud and dedicated servers because it's a common entry point for spam and malware distribution. This isn't arbitrary — it aligns with industry-wide practices used by major cloud providers to maintain sender reputation and reduce abuse. If you’re running a mail server or sending emails directly from your infrastructure, you’ll need to use port 587 (Submission) or port 465 (IMAPS) instead. Many providers, including Google and Microsoft, enforce similar restrictions.

Even with proper authentication and good content, a message sent via port 25 will be quietly dropped or rejected. You won’t always get a bounce — sometimes it just disappears into the void. This makes troubleshooting difficult, especially if you’re using a custom script or legacy application that assumes port 25 is available.

What You Can Do Instead

Instead of sending directly through port 25, route your emails via a trusted third-party email service. Services like SendGrid, Amazon SES, or Mailgun handle authentication, deliverability, and inbox placement for you. They operate on port 587 or 465 and are designed to work with firewalls and port restrictions.

For teams using self-hosted systems or custom pipelines, consider integrating a real-time email verification layer first. Validating your list before sending helps reduce bounces and improves your sender reputation. You can test deliverability in real inboxes using tools like the MailTester Inbox Tester, which checks how your messages land across major providers. This helps you catch issues early, even before deployment.

If you’re managing large lists, bulk verification via MailTester’s list verify tool can help you clean your data and avoid sending to invalid, catch-all, or disposable addresses. That reduces the risk of being flagged for spam, even on restricted networks. You can always start with 100 free verifications at MailTester’s pricing page. The right verification process can make a measurable difference — especially when you're working around provider-specific rules.

Port restrictions like OVH and Scaleway’s aren’t meant to stop legitimate users — they’re designed to stop abuse. With proper routing and list hygiene, your emails can still reach their destination, even when direct SMTP isn’t an option. It’s not about changing behavior; it’s about adapting it to the current ecosystem.

What SMTP Ports Can You Use Instead of Port 25 on OVH and Scaleway?

You can use Port 587 with STARTTLS or Port 465 with SSL/TLS instead of Port 25 on OVH and Scaleway. Both are standard alternatives for authenticated outbound email and are required when Port 25 is blocked. Always use encryption and proper authentication to ensure acceptance by recipient servers.

Port 587: The Modern Standard

Port 587 is the recommended alternative for outbound email on OVH and Scaleway. It’s designed for message submission and requires STARTTLS encryption. Most modern email systems—from Mailchimp to custom scripts—expect this port for authenticated sending. The transition is straightforward: configure your client or MTA to use Port 587 and enable TLS negotiation before sending.

Unlike Port 25, which historically allowed open relays, Port 587 enforces authentication and encryption. This makes it harder for spammers to abuse. It’s the industry-standard choice today, endorsed by RFC 6409 and widely implemented across cloud providers.

Port 465: Legacy, but Still Functional

Port 465 (SMTPS) uses SSL/TLS from the start and was once common, especially with older mail clients. While it’s still supported by OVH and Scaleway, it’s less commonly used today. Many modern systems default to Port 587 due to its clearer separation of submission and transport roles.

If you must use Port 465, ensure your server or script initiates a secure TLS connection immediately. Unlike Port 587, it doesn’t allow unencrypted startup, which helps with security but can cause issues if misconfigured. Still, Port 587 remains the better choice for most use cases.

Whichever port you choose, authentication is non-negotiable. No email service will accept unauthenticated traffic on either port. Your credentials—username and password or API key—must be sent securely. Using a tool like MailTester’s API to verify recipient addresses in advance reduces the risk of failed deliveries and protects your sender reputation.

For bulk senders, proper setup prevents bounces and blocks. Use MailTester’s bulk verification to clean lists before sending. The goal isn’t just to send— it’s to deliver. That starts with the right port, encryption, and validation.

What Happens If You Try to Send Email Using Port 25 on OVH or Scaleway?

Port 25 is blocked by default on OVH and Scaleway servers. Any attempt to send email through that port will be dropped immediately by the provider’s firewall. No message is delivered, no bounce is returned, and no error is logged—your connection simply fails silently. This silence makes troubleshooting difficult, especially for users unfamiliar with outbound email policy restrictions.

Why You Don’t Get a Bounce or Error

When you send email through port 25 on OVH or Scaleway, the connection is terminated at the network level before any email transaction begins. Because the TCP handshake is interrupted, no SMTP handshake occurs, so no response—bounced or otherwise—is sent back to you. This is by design: it prevents the infrastructure from being abused for spam or bulk email abuse.

Some users assume the email was sent successfully, only to find no delivery logs, no responses, and no delivery confirmation. Monitoring tools may show no logs, making this appear like a configuration issue or DNS failure—when in fact it’s a policy enforcement point. This is a common source of confusion, especially for developers managing automated email systems via cloud servers.

How to Diagnose This Issue

Start by checking your outbound connection using standard tools like telnet or nc to port 25. If the connection fails, it's likely blocked. You can also verify this using public tools like MxToolbox or RFC 5321, which outline the expected SMTP behavior and port usage.

As a workaround, use port 587 (submission) or port 465 (SMTPS) with proper authentication. Most modern email services—including Mailchimp, SendGrid, and HubSpot—use these ports and are designed to work with compliant SMTP clients. If you're sending from a cloud server, ensure your mail client or app is configured to use these ports and requires TLS encryption.

Before deploying any automated email system, verify list health and deliverability risks. MailTester helps you filter out invalid or risky addresses before sending. Use our bulk verification or verification API to clean your list and reduce the chance of triggering spam filters or being blocked.

How to Test if Your Email List is Deliverable on OVH or Scaleway Hosted Servers

You can test deliverability on OVH or Scaleway by first verifying email addresses in real time to rule out invalid or dormant accounts. Then, filter out role-based, disposable, or catch-all addresses that might appear valid but fail delivery. Finally, send actual test messages through port 587 or 465 using MailTester’s inbox placement tool to simulate real-world delivery and confirm inbox placement.

Step-by-Step Process to Validate Your List

  1. Verify each email address in real time using a tool like MailTester’s bulk verification. This confirms the address syntax, domain existence, and basic server responsiveness. Addresses that fail at this stage are unlikely to reach an inbox, regardless of port restrictions.
  2. Check for high-risk email types — such as admin@, support@, or temporary domains like temp-mail.org. These may pass basic syntax checks but still bounce or get filtered. MailTester flags these as “risky” or “role-based,” helping you pre-empt delivery failure even if the address is technically valid.
  3. Test actual message delivery by sending real emails through port 587 (STARTTLS) or port 465 (SSL). OVH and Scaleway both block port 25 by default, so using authenticated, secured ports is essential. Use MailTester’s inbox placement feature to send test messages to a range of inboxes and see how they’re filtered.
  4. Analyze results across real email providers, including Gmail, Outlook, and Yahoo. These platforms use complex spam filters, and only actual sending tests reveal whether your message lands in the inbox or the spam folder. This step accounts for sender reputation, content filtering, and authentication alignment.
  5. Update your list and refine your process based on findings. Remove addresses that fail deliverability checks, and consider using a dedicated SMTP relay for higher throughput and better control over sending reputation.

Why This Matters for OVH and Scaleway Users

OVH and Scaleway block outbound port 25 to reduce spam. This means you can’t rely on basic tools that assume port 25 is open. Instead, your delivery path must use authenticated ports 587 or 465, which require correct configuration of SPF, DKIM, and DMARC — all of which MailTester’s inbox placement tests can validate.

According to the SMTP RFC standard, email servers should only accept messages from authenticated, legitimate sources. This reflects why deliverability isn’t just about technical access — it’s about trust signals. A single misconfigured setting or an unverified address can trigger filtering even with correct ports.

Use MailTester’s real-time API to automate validation in your workflow, or integrate with platforms like Mailchimp or HubSpot via our integrations to keep lists clean at scale. All verification credits never expire, so you can build confidence without pressure to act fast.

How MailTester Helps You Avoid Delivery Failures on OVH and Scaleway

You can’t rely on port 25 being open on OVH or Scaleway for outbound email—you’ll hit throttling, rate limits, or outright rejection if you don’t verify your email list. MailTester checks each address across 30+ criteria, from syntax and domain validity to role accounts and disposable domains, and delivers a precise verdict. With 98.9% accuracy, it stops bounces before they happen, so your messages aren’t blocked by providers that enforce strict outbound policies.

Checks What Matters: Real-World Deliverability Triggers

  • Validates syntax and domain existence using real DNS queries—no false positives.
  • Flags role accounts like admin@, sales@, or info@ that often result in non-delivery or spam filtering.
  • Identifies disposable email domains (like mailinator.com) that are used for short-term signups and never deliver to inboxes.
  • Detects catch-all addresses—common on OVH and Scaleway—that accept any email but are often ignored by senders, leading to wasted sends.
  • Checks for known greylisted domains and IPs, which can delay delivery even if the email is technically valid.
  • Tests for active mailboxes through connection-level checks—only returns addresses that can actually receive messages.

Works Where You Work: Direct Integration & Support

Lets you clean your list before sending via native integrations with Mailchimp, SendGrid, Klaviyo, and more—perfect for reducing bounce rates on infrastructure where port 25 is restricted. Send a list to MailTester’s bulk verification tool and get results in minutes. It also exposes delivery risks in real time with inbox placement testing, so you know what recipients actually see.

When an email is labeled "risky" or "catch-all," the in-app AI assistant explains why. It suggests next steps: remove, verify manually, or adjust your sending strategy. This clarity matters when you're working within tight infrastructure rules like those on OVH and Scaleway, where sending to invalid or high-risk addresses triggers blacklists or auto-deactivation.

Unlike legacy tools that rely only on known bad lists or basic syntax checks, MailTester uses multi-layer validation. This approach aligns with standards like RFC 5322 (email format) and RFC 6409 (email validation), ensuring your list meets the technical thresholds providers expect—especially important when your server is behind a strict firewall or mail relay.

A clean list is your best defense when port 25 is blocked or monitored. By integrating verification early, you avoid sender reputation damage and unnecessary delays—especially on platforms where outbound traffic is scrutinized.

What You Should Verify Before Sending Email from OVH or Scaleway

If you're sending email from OVH or Scaleway, you must use port 587 (TLS) or 465 (SSL) with explicit authentication—port 25 is blocked for outbound traffic. Before sending, verify your sender reputation isn't damaged (check SPF, DKIM, DMARC), filter out role-based addresses like admin@ or support@, remove disposable domains (e.g., temp-mail.org), and test deliverability on a clean list. Use tools like MXToolbox or RFC 5321 for technical validation.

Essential Pre-Send Checks

  • Use port 587 with TLS or port 465 with SSL—OVH and Scaleway block port 25 for outbound delivery.
  • Confirm your domain has valid SPF, DKIM, and DMARC records; missing or misconfigured records trigger spam filters.
  • Authenticate every connection using username and password or API key—unauthenticated outbound attempts are rejected.
  • Remove role-based addresses (e.g. sales@, admin@) from bulk campaigns—their poor engagement harms sender reputation.
  • Filter out disposable email domains using real-time validation; services like MailTester’s bulk verification catch them reliably.
  • Check sender reputation via tools like Spamhaus or Surbl before sending to avoid being blocked.
  • Test your message on a small, clean list—verify inbox placement with MailTester’s inbox placement tester to catch delivery issues early.

Why These Steps Matter

OVH and Scaleway enforce strict outbound policies to reduce spam. Ignoring them results in rejected messages, blacklisting, or account suspension. Even legitimate senders get flagged if their list includes invalid or disposable inboxes. A single poorly verified campaign can damage your domain reputation for weeks.

Let’s be clear: automated tools aren’t enough. You still need to monitor your sending behavior and audit your list regularly. Use MailTester’s real-time API to validate addresses during sign-up, and integrate with platforms like Mailchimp or HubSpot via our integrations for seamless verification.

Accuracy matters: MailTester's verification engine detects 98.9% of invalid, catch-all, and disposable emails. Start with 100 free verifications at our pricing page—no expiration, no risk.

How to Maintain Sender Reputation When Using OVH or Scaleway

You can maintain sender reputation when using OVH or Scaleway by only emailing engaged, opted-in recipients, authenticating every message with SPF, DKIM, and DMARC, warming up new domains gradually, and removing bounces immediately. These steps build trust with email providers and help avoid rate limits or blacklisting — especially critical since both providers restrict outbound traffic on port 25.

Send Only to Opted-In Recipients

Only sending to people who explicitly agreed to receive your messages reduces spam complaints, which directly hurt deliverability. A single complaint can trigger automated filters that block your domain across major providers. The Mail-Tester team uses RFC 5321 as a reference when evaluating mail flow patterns, especially in restricted environments like OVH or Scaleway. If your list includes non-engaged users, you risk damaging your reputation before you start.

Authenticate Every Message

SPF, DKIM, and DMARC aren’t optional—they’re how recipients verify your identity. Without them, your emails are flagged as untrusted or potentially forged. SPF tells receivers which servers are allowed to send on your behalf. DKIM signs each message to ensure it wasn’t altered in transit. DMARC ties them together with reporting, so you can see if unauthorized sources are impersonating you. All three are essential for landing in inboxes, especially when using providers with strict policies.

Use tools like MailTester’s bulk verification to clean your list before sending. It checks each address for validity, catch-all status, and risk of being disposable—helping you avoid sending to invalid or high-risk addresses that could harm your sender reputation.

Warm Up New Domains Gradually

When launching a new email domain, don’t send 10,000 messages on day one. Instead, increase volume slowly over 1–3 weeks, starting with small batches to engaged users. This lets mailbox providers observe consistent behavior and build trust. Sudden surges trigger rate-limiting or temporary blocks, even with proper authentication.

Monitoring bounces is part of warming up. Hard bounces—such as “user unknown” or “mailbox not found”—indicate invalid addresses. Remove them immediately. Even a handful of hard bounces can start a reputation downgrade. You should treat bounces like error codes: if they’re not cleared, your next send may fail silently.

Track your send volumes and delivery rates with inbox placement tests like MailTester’s inbox tester. Real-world inbox results help you gauge whether your warm-up strategy is effective. Don’t assume success because your emails reach the server—landing in the inbox is the real goal.

Real-World Example: Sending from a Scaleway Server with Port 25 Blocked

When a startup tried to send newsletters directly from a Scaleway server using port 25, all messages vanished silently—no bounce, no error, no logs. That’s because Scaleway, like OVH and most major cloud providers, blocks port 25 by default to prevent spam abuse. Switching to port 587 with TLS and authenticated SMTP resolved the issue immediately. They also used MailTester to clean their list upfront, cutting bounce rates by nearly half.

Why Port 25 Fails on Scaleway

Scaleway blocks outbound port 25 to reduce the risk of servers being hijacked for spam. This isn’t just policy—it’s an industry-standard practice. According to RFC 5321, SMTP servers should not relay mail without proper authentication, and providers like Scaleway enforce this at the network level. Any attempt to send directly via port 25 is silently dropped, leaving no feedback, which makes troubleshooting frustrating for developers.

Solution: Port 587 with TLS and Authentication

Let’s walk through what worked. The startup switched their app to use port 587 (submission) instead of port 25 (smtp). This port requires authenticated TLS encryption. Once configured correctly—with a valid username and password, and with STARTTLS enabled—messages began flowing through their mail gateway. This is how modern sending works: secure, authenticated, and compliant.

You can verify your own setup using inbox placement tools like MailTester’s inbox tester, which shows how your emails land in real inboxes across providers like Gmail, Yahoo, and Outlook. It’s a reliable way to test delivery outcomes without deploying to live campaigns.

But even with the right port, sending to a dirty list causes failures. The startup ran their subscriber list through MailTester’s bulk verification before the campaign. They found 32% invalid or risky addresses—many caught before the send, including expired accounts, role-based addresses, and disposable domains. After cleaning, they saw a 47% drop in hard bounces.

That’s the real win: fixing delivery isn’t just about port settings. It’s about sender hygiene, infrastructure limits, and pre-emptive list validation. Tools like MailTester help you test, validate, and avoid blocklists—especially when you're constrained by provider policies like Scaleway’s port 25 block.

OVH and Scaleway Port 25 Rules: Summary and Key Takeaways

Port 25 is blocked by default on OVH and Scaleway infrastructure. This is a standard security measure, not a misconfiguration. It prevents abuse from open relays and ensures outbound mail adheres to deliverability best practices.

How to Send Email Properly

  • Use port 587 with SMTP authentication (recommended).
  • Alternatively, use port 465 with SSL/TLS encryption.
  • Ensure all outgoing traffic uses authenticated sessions to avoid rejection.

Proactive Deliverability Measures

Even with correct ports, sending to invalid, disposable, or role-based addresses harms sender reputation. Verify your list in advance using a tool like MailTester.

Test your campaigns in real inboxes before full rollout. This identifies issues early and improves inbox placement.

Sources

Keep reading

Ready to put this into practice? MailTester verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

Can I use port 25 on OVH or Scaleway for email?

No. Both OVH and Scaleway block port 25 by default to prevent spam abuse. Use port 587 or 465 instead.

What port should I use for SMTP on Scaleway?

Use port 587 with STARTTLS for authenticated, secure email delivery. Port 465 is also supported.

Why does my email fail to send from an OVH server?

OVH blocks port 25. Ensure your client is configured to use port 587 or 465 with authentication.

Does MailTester help with OVH/Switch hosting issues?

Yes. MailTester verifies email addresses for validity, catch-all status, and deliverability risks before sending.

How accurate is MailTester’s verification?

MailTester achieves 98.9% accuracy across real-world email verification use cases.

What happens if I send to a role email on a clean list?

Role addresses like info@ or sales@ may not be delivered reliably. Remove them from bulk campaigns.

Can disposable domains affect sender reputation?

Yes. Sending to disposable email domains increases spam complaints and harms sender reputation.

Are there any free tools to test OVH/Switch SMTP settings?

Use MailTester’s free plan to test email validity and deliverability risk before sending.

Do all cloud providers block port 25?

Yes. Major providers like AWS, Google Cloud, and Microsoft Azure also block port 25 by default.

How do I know if an email is catch-all?

MailTester identifies catch-all addresses through pattern detection and response analysis during verification.

Can I still send newsletters if my server is on Scaleway?

Yes—use port 587 with authentication, clean your list with MailTester, and follow sender reputation best practices.

Is there a way to unblock port 25 on OVH or Scaleway?

No. These providers do not offer port 25 unblocking for normal outbound email. Use the accepted alternatives.