Microsoft 365 4.5.3
Too many recipients
Microsoft 365 4.5.3 means: too many recipients. It is a temporary failure, so your server retries it. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Too many recipients
Possible cause
The message has more than 200 SMTP envelope recipients from the same domain.
Provider's advice
An envelope recipient is the original, unexpanded recipient that's used in the RCPT TO command to transmit the message between SMTP servers. If Microsoft 365 or Office 365 returns this error, the sending server must break up the number of envelope recipients into smaller chunks (chunking) and resend the message.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 4.5.3: Too many recipients |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
In practice
Microsoft counts envelope recipients from the same domain, not addresses in the To header. Exchange Online retries the remaining recipients automatically when the message is split. If you send with a custom client, keep each transaction under 200 recipients per domain.
What the sender should do
The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Would verifying the list have prevented Microsoft 365 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.
Common questions
- What does Microsoft 365 4.5.3 mean?
- Microsoft 365 4.5.3 means: too many recipients. It is a temporary failure, so your server retries it. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Should I retry after Microsoft 365 4.5.3?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Microsoft 365 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.
The same code elsewhere
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (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