SMTP error codes and bounce messages

550 5.7.1 — Message refused — delivery not authorizedThe receiving server refused the message because of a policy: your IP or domain reputation, a blacklist listing, failed authentication, or content filtering. The text after the code tells you which. 550 5.7.26 — Unauthenticated email — SPF / DKIM / DMARC failedGmail and others use 5.7.26 when a message fails authentication: it did not pass SPF or DKIM, or it failed DMARC for the From domain. Since 2024 Gmail and Yahoo reject much more unauthenticated mail. 550 5.7.25 — Reverse DNS (PTR) check failedThe receiver could not confirm your sending IP with reverse DNS: the IP has no PTR record, or the PTR name does not resolve back to the same IP. 550 5.7.23 — SPF validation failedThe sending IP is not authorized by the SPF record of the envelope sender domain, and the receiver refused the message. 554 5.7.1 — Transaction failed — rejected by policyA permanent rejection for policy reasons, similar to 550 5.7.1: spam filtering, blocklists, relay denied or a recipient rule. 554 5.7.5 — Permanent error evaluating DMARC policyThe receiver could not evaluate your authentication because a record is broken — for example an SPF PermError (too many lookups or syntax) or a malformed DMARC record. 530 5.7.0 — Authentication requiredThe server you connected to requires you to log in (SMTP AUTH) or to start TLS before sending. 535 5.7.8 — Authentication credentials invalidThe SMTP login failed: the username or password was rejected. 554 5.7.0 — Rejected for policy reasonsA broad security / policy rejection. Read the message text: it often names a blocklist, spam score or authentication problem. 550 5.7.606–649 — Microsoft 365: banned sending IPMicrosoft 365 (Exchange Online) blocked your sending IP because of its reputation. The bounce links to Microsoft's delisting portal at sender.office.com. (Outlook.com / Hotmail blocks use S3140 / S3150-style messages instead.)

550 5.1.1 — User unknown — the mailbox does not existThe receiving server says the recipient address does not exist. This is a permanent failure (hard bounce); retrying will not help. 550 5.1.0 — Address rejected — sender or recipient refusedA generic address error. Depending on the wording it refers to the recipient (address not accepted) or to your sender address. 550 5.1.2 — Bad destination domain — host not foundThe domain after the @ does not exist or has no mail server, so the message cannot be routed. 550 5.4.1 — Recipient address rejected: access deniedIn RFC 3463, 5.4.1 means "no answer from host", but Microsoft 365 (Exchange Online) reuses it for "Recipient address rejected: Access denied" — usually the mailbox does not exist or directory-based edge blocking is on. 552 5.3.4 — Message too bigThe message, including attachments, is larger than the receiving server accepts. 553 5.1.3 — Invalid address syntaxThe address is not written in a valid format, so the server will not accept it. 4.4.7 — Delivery time expiredYour own server gave up after retrying for a long time (often 1–5 days) because it kept getting temporary errors or could not connect. 5.4.4 — Unable to route — no mail server foundThe sending server could not find where to deliver: the recipient domain has no usable MX or A record.

552 5.2.2 — Mailbox full / over quotaThe recipient's mailbox has no space left. Some servers return this as a temporary 452 4.2.2, others as a permanent 552. 421 4.7.0 — Temporarily deferred — try again laterA temporary refusal. The receiver is slowing you down — often because of reputation, unusual volume, or missing authentication. Your server will retry automatically. 421 4.7.28 — Unusual rate of unsolicited mail (Gmail rate limit)Gmail detected an unusual rate of mail from your IP or domain that looks unsolicited and is temporarily limiting it. 451 4.7.1 — Greylisted / temporarily rejectedThe receiver temporarily refused the message, often as greylisting: unknown senders are asked to retry, and real mail servers get through on the next attempt. 450 4.2.1 — Mailbox temporarily unavailableThe mailbox cannot receive mail right now — it may be temporarily disabled, receiving too much mail, or rate limited. 452 4.2.2 — Mailbox full (temporary)The recipient's storage is full. The server will keep retrying for a while.