Spam filter · 4.x.x transient failure
Every SMTP 421 response on record from Exchange Online Protection, with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender.
smtp;421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel. example.com. PRX8 [example.com]
You are sending faster than this receiver currently accepts from you. Slow down and the automatic retry will deliver the queued mail.
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender. At Exchange Online Protection, it is a temporary deferral that is retried automatically, and the specific reason is in the enhanced status code and message text of each response below.
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Exchange Online Protection's postmaster documentation.
