MailTester helps you understand how your emails are treated by major mailbox providers like Gmail, Outlook, and Yahoo. This hub focuses on inbox placement—whether your message actually arrives in the user’s inbox, not the spam folder. Different providers use unique filtering rules based on content, sending practices, and sender reputation. Gmail, for example, employs SpamAssassin scoring and internal risk detection, while Outlook and Yahoo rely on their own algorithms and feedback loops. Corporate and enterprise email systems, like Microsoft 365, apply additional rules that can affect deliverability.

You’ll find guidance on how technical factors like message size, sender reputation, authentication (SPF, DKIM, DMARC), and content signals impact inbox placement. Some issues stem from provider-specific thresholds—like Gmail’s 102KB limit or SpamAssassin’s 5.0 score cutoff—which can block or tag your messages. Other problems come from recipient-specific rules, such as those used by Orange.fr, Proton Mail, or UOL in Brazil.

What you'll find in this hub

  • How Gmail’s SpamAssassin thresholds and SCL scores affect deliverability
  • Postmaster tools and reputation metrics for Gmail, Outlook, and Yahoo
  • Steps to diagnose why emails land in spam or are blocked
  • Tools and services to verify inbox placement across providers
  • Common issues with bulk senders and corporate email filters
  • How to use feedback loops and validation services to improve inbox delivery

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Browse all 156 articles in Inbox placement by mailbox provider: Gmail, Outlook, Yahoo and spam filters →

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