Glossary
SMTP 452
A transient failure meaning the receiving system has insufficient storage or resources. It is also the reply used when a sender issues too many RCPT commands in one transaction.
In plain terms
They are out of room, or you asked for too many recipients at once.
Why it matters for deliverability
Two very different causes share this code. A full mailbox is about one recipient; a RCPT limit is about how you are batching, and it will keep happening until you change the batch size.
What to do about it
- If it is per-recipient, retry — mailboxes get emptied.
- If it appears after many RCPT commands on one connection, reduce recipients per transaction.
- Watch for repeated 452s on the same address over weeks; a permanently full mailbox is a disengaged subscriber.
Source
RFC 5321 — Simple Mail Transfer Protocol
“452 Requested action not taken: insufficient system storage”
Checked 2026-08-29
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Definitions reviewed 2026-08-29. Spotted something wrong? Tell us.
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