4.5.3 Microsoft bounce: too many recipients
What 4.5.3 means
The message has more than 200 envelope recipients from the same domain, so Exchange Online asks the sender to split it.
Message text
Too many recipients
Common causes of 4.5.3
- More than 200 SMTP envelope recipients from one domain in a single message
How to fix 4.5.3
- Split the message into smaller chunks of recipients per domain and send each chunk separately.
- Check the sending software's recipient limit per message.
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How to read 4.5.3
The second part, 4.5.3, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 4 is a Persistent transient failure (4xx): the command was not accepted, but the condition may clear, so the sender should try again later (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 5 is mail delivery protocol: a problem in the SMTP conversation itself. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 4.4.7 Microsoft: message expired
- 4.7.26 Microsoft: ipv6 needs spf or dkim
- 432 4.3.2 Microsoft: recipient thread limit
FAQ
What does 4.5.3 mean?
The message has more than 200 envelope recipients from the same domain, so Exchange Online asks the sender to split it. This is a temporary failure.
How do I fix 4.5.3?
Split the message into smaller chunks of recipients per domain and send each chunk separately. Check the sending software's recipient limit per message.
Is 4.5.3 permanent, and should I retry?
4.5.3 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.