SMTP error codes, explained
Paste a bounce message to find out why your email didn't arrive, or browse the most common codes below.
What the numbers mean
550 The reply code
The first number. 2xx means it worked, 4xx means try again later, and 5xx means it failed for good.
5.1.1 The status code
A more detailed status. The middle number says what kind of problem it is: 1 is the address, 2 the mailbox, 3 the mail system, 4 the network, 5 the protocol, 6 the content and 7 security or policy.
The text after it
Each server writes its own explanation, and it often says more than the numbers. Look for a reason, the name of a blocklist, or a link.
Security and policy
- 550 5.7.0 Rejected by spam filter
- 550 5.7.1 Delivery not authorized, message refused
- 554 5.7.1 Client host rejected or relay access denied
- 550 5.7.23 SPF validation failed
- 550 5.7.25 Reverse DNS validation failed
- 550 5.7.26 Unauthenticated email or DMARC failure
- 550 5.7.509 Sending domain does not pass DMARC
- 550 5.7.606 Access denied, banned sending IP
Bounce questions
What's the difference between a hard and a soft bounce?
A hard bounce is a permanent failure, usually a 5xx code such as an address that doesn't exist. The sender's server gives up straight away. A soft bounce is temporary, usually a 4xx code such as a full mailbox or a busy server, and the sender's server keeps retrying for a few days before giving up.
What is a suppression list?
Most newsletter and email sending services keep a list of addresses that have hard bounced, and stop sending to them to protect their reputation. If your address bounced even once, a sender may keep skipping it after it starts working again. Ask them to remove it from their suppression list, or give them a new address.
The address is right, so why did my email bounce?
Plenty of bounces have nothing to do with the address. The receiving server might think the email is spam, the sender's domain might be missing SPF, DKIM or DMARC records, the mailbox might be full, or the sending server might be on a blocklist. The status code and the text after it tell you which.
Who has to fix it?
It depends on the code. Address and mailbox problems (5.1.x and 5.2.x) are usually fixed by the recipient. Authentication and policy problems (5.7.x) are almost always fixed by the sender or their email provider. Each page above says who needs to do what.
A different email address for every website
addy.io gives you unlimited aliases that forward to your real inbox. Turn one off the moment it gets spam.
Get started free