SMTP 421 — transient failure retry strategy
A 421 response is a temporary failure: the receiving server is asking you to retry later. The right retry policy protects your sender reputation and your recipients' patience.
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SMTP errors
Explanations of SMTP response codes, retry decisions, and delivery failures, with diagnostic steps for common temporary and permanent errors.
SMTP 550 — permanent failure codes and what to do with each
A 550 response means the mailbox rejected the message and is asking you not to retry. Decode the common sub-codes and the right operational response for each.