Microsoft 365 421 4.3.240
The maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel
Microsoft 365 421 4.3.240 means: the maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel. 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
The maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel.
Possible cause
The sender opened too many simultaneous authenticated connections to the service, and exceeded the allowed limit of 250.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | 421: <domain> Service not available, closing transmission channel (RFC) |
| Enhanced code | 4.3.240: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.3.0 other or undefined mail system status. |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
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 365 421 4.3.240?
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 365 421 4.3.240 mean?
- Microsoft 365 421 4.3.240 means: the maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel. 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 365 421 4.3.240?
- 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 365 421 4.3.240?
- 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
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (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