SMTP Error Codes Explained: 421, 535, 550, 554 and More

When an email can't be delivered, the bounce message (also called a non-delivery report, NDR or DSN) contains a short code such as 550 5.1.1. That code tells you whether the problem is temporary or permanent, whose side it's on, and usually exactly what to fix. This guide explains how to read the codes and lists the ones Gmail, Microsoft 365, Outlook.com and Yahoo return most often.

How to read an SMTP error code

Most bounces show two codes side by side:

550 5.1.1 The email account that you tried to reach does not exist.

• The three-digit reply code (550) is the basic SMTP answer. The first digit is what matters most: 4 means a temporary failure, 5 a permanent one.
• The enhanced status code (5.1.1) adds detail in three parts: class.subject.detail (RFC 3463).

The middle number of the enhanced code says where the problem is:

The most common codes and what to do

Gmail and Google Workspace

Gmail's bounces always include the reason in plain words after the code, and many of them point to the same three causes:

• Authentication. 550 5.7.26 means your domain's SPF, DKIM or DMARC didn't hold up. Since 2024, Gmail requires SPF or DKIM from every sender, and all three from anyone sending 5,000 or more messages a day.
• Reputation and rate. 421 4.7.0 and 550 5.7.1 with “very low reputation”, or 421 4.7.28 “unusual rate”, mean Gmail is treating the traffic as suspicious. Check for a compromised account or a contact form being abused before anything else.
• Message format. Several 550 5.7.1 variants reject messages that aren't RFC 5322 compliant — no valid From address, more than one From header, a missing Message-ID, or duplicate headers. These come from website code or scripts, not from normal mail apps.

If you're the one who can't send through smtp.gmail.com and you see 535 5.7.8 “Username and Password not accepted”, see Gmail SMTP settings for the sign-in options.

Microsoft 365 and Outlook.com

Microsoft's non-delivery reports use their own detailed enhanced codes:

Yahoo

Yahoo groups its errors rather than giving one meaning per code:

• 421 or 451 — a temporary problem; retry later. Messages marked TS01, TS02 and similar are temporarily deferred, typically because of complaints, spam-like content, poor IP reputation or unusual traffic.
• 553 or 554 — a permanent problem; don't retry. Causes include failing DKIM or DMARC, an IP listed by Spamhaus, unacceptable content, or a recipient that doesn't exist — for example “554 delivery error: dd This user doesn't have a yahoo.com account”.

What to do with a bounce

1. Look at the first digit. A 4 usually fixes itself; wait a few hours before doing anything. A 5 won't.
2. Read the text after the code. Gmail and Microsoft both say the reason in plain words.
3. Check the address for any 5.1.x code.
4. Check your domain's records for any authentication-related 5.7.x code, with the SPF, DKIM & DMARC Checker.
5. If the bounce says your own mailbox or account is blocked, change your password and check for forwarding rules you didn't create — compromised accounts are the most common cause.