X.5.3 · Mail delivery protocol
SMTP enhanced status code 4.5.3
Too many recipients
X.5.3 means too many recipients were given for one message. Servers expect the sender to split the message and send the rest in a later transaction.
| Registry code | X.5.3 |
|---|---|
| Forms in use | 4.5.3 |
| Registry name | Too many recipients |
| Subject | 5: Mail delivery protocol |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed |
| Associated basic codes | 451 |
| Defined in | [RFC 3463] |
What the registry says
More recipients were specified for the message than could have been delivered by the protocol. This error should normally result in the segmentation of the message into two, the remainder of the recipients to be delivered on a subsequent delivery attempt. It is included in this list in the event that such segmentation is not possible.
IANA SMTP Enhanced Status Codes registry, X.5.3
The basic codes it travels with
- 451 Requested action aborted: local error in processing
In practice
Providers cap recipients per message: Gmail returns 452 4.5.3 for too many recipients and Microsoft 4.5.3 when a message has more than 200 envelope recipients from the same domain. Well-behaved servers split the message into several transactions. If your tool does not, send in smaller batches.
What the sender should do
Send in smaller batches of recipients per message, within the receiver's published limit.
Would verifying the list have prevented 4.5.3?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
How providers use X.5.3
Quoted from each provider’s own error reference.
- Gmail 452 4.5.3
Domain policy size per transaction exceeded, please try this recipient in a separate transaction. For more information, go to Message bounced due to a policy rule.
- Gmail 550 5.5.3
Too many recipients for this sender. For more information, go to About SMTP error messages and review RFC 5321 specifications.
- Microsoft 365 4.5.3
Too many recipients
Common questions
- What does 4.5.3 mean?
- X.5.3 means too many recipients were given for one message. Servers expect the sender to split the message and send the rest in a later transaction.
- What should I do about 4.5.3?
- Send in smaller batches of recipients per message, within the receiver's published limit.
- Would an email verifier have caught 4.5.3?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05