Enhanced SMTP status code · RFC 3463 · IANA
4.3.240 The maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel
The sender opened too many simultaneous authenticated connections (the limit is 250).
- Class: Temporary failure
Class 4: a persistent transient failure. The message was fine, but a passing condition delayed or stopped delivery; a later attempt may succeed (RFC 3463 §3.1). The sending server retries on its own for a while.
- Subject: X.3 · Mail system
Something about the destination server; under its administrator's control.
How 4.3.240 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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421 4.3.240
The maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel.
The sender opened too many simultaneous authenticated connections (the limit is 250).
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026