Spam filter · 4.x.x transient failure
Every SMTP 421 response on record from Proofpoint, 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 4.1.1 <[email protected]>: Recipient address rejected: unverified address: host example.com[x.xx.xx.xx] said: 450 4.1.1 <[email protected]>: Recipient address rejected: unverified address: lost connection with mail.example.com[x.xx.xx.xx] while sending RCPT TO (in reply to RCPT TO command)
The receiver could not confirm the recipient address on this attempt. If the retry keeps failing, treat the address as invalid and remove it.
smtp;421 4.1.1 <[email protected]>: Recipient address rejected: unverified address: Address verification in progress
The receiver could not confirm the recipient address on this attempt. If the retry keeps failing, treat the address as invalid and remove it.
Postmaster & documentation
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 Proofpoint, 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 Proofpoint's postmaster documentation.
