Reference

SMTP response reference

What the responses in your mail log actually mean, whether retrying will help, and what to change so they stop. Written from operating these systems, not from the spec.

14 responses documented. Categories and recommended actions match smtpsift.

4.2.2
4.2.2 means the recipient's mailbox is over quota. It is temporary and the address is still valid, so retry. If it persi...4.4.1
4.4.1 means your server could not complete a connection to the receiving host. It is a transport problem rather than a p...5.1.1
5.1.1 means the mailbox does not exist at the receiving domain. It is permanent. Suppress the address immediately. Conti...5.2.1
5.2.1 means the mailbox exists but is disabled, suspended or not accepting mail. It is permanent. Suppress it, and treat...5.7.1
5.7.1 is a policy rejection. The receiving server understood the message and refused it on purpose. The cause sits in th...spamhaus
This means the receiving server queried Spamhaus, found your sending IP listed, and refused the connection. Delisting is...tls-handshake
A TLS handshake failure means the connection opened but encryption could not be negotiated. Occasional failures are norm...4.7.28 · Gmail
Gmail 4.7.28 is a rate limit, not a block. Gmail is telling you it has seen an unusual volume of mail from your IP or do...5.7.1 · Gmail
Gmail 5.7.1 is a hard block. Unlike 4.7.28, this is Gmail refusing the message outright, usually on content or on the re...5.7.26 · Gmail
Gmail 5.7.26 means the message failed authentication. Gmail requires bulk senders to pass SPF or DKIM with domain alignm...4.7.500 · Microsoft / Outlook
Microsoft 4.7.500 is throttling. The message has not been rejected, it has been deferred because Microsoft is limiting h...5.7.606 · Microsoft / Outlook
Microsoft 5.7.606 means your sending IP is on Outlook's block list. It is a reputation block, not a content or authentic...S3140 · Microsoft / Outlook
S3140 is Microsoft's internal reason code attached to a block, and it points at the reputation of the sending IP. You wi...TS03 · Yahoo
Yahoo's [TS03] tag marks a deferral for policy or reputation reasons. It arrives as a 4xx, so the message is still queue...