How to read an SMTP error
A bounce line usually has three parts. Take 550 5.7.26 This email has been blocked because the sender is unauthenticated. as an example:
- The reply code (
550). Defined in RFC 5321. The first digit is what matters most: 4 means try again later, 5 means the message was refused. - The enhanced status code (
5.7.26). Defined in RFC 3463 and listed in the IANA registry. It adds a subject and a detail. - The text. Written by the receiving server, often with a link to the provider's help pages. This is where the specific reason lives.
What the middle number of the enhanced code points to, using the subjects from RFC 3463:
- X.1.x, addressing: a wrong or unknown address.
- X.2.x, mailbox: the recipient's mailbox, such as a full quota.
- X.3.x, mail system: the destination system, such as a size limit.
- X.4.x, network and routing: DNS, routing, loops and timeouts.
- X.5.x, mail delivery protocol: commands the server didn't accept.
- X.6.x, message content or media: malformed or unsupported content.
- X.7.x, security or policy: authentication, reputation, blocklists and policies.
Temporary errors: let the queue work
A 4xx reply doesn't bounce the message. Your server keeps it and retries. RFC 5321 recommends waiting at least 30 minutes between attempts and not giving up for at least four to five days. A single deferral is normal; a deferral that repeats for hours is a signal to read the text and fix what it names.
Permanent errors: fix, then resend
A 5xx reply means the message was refused. Sending it again unchanged will fail again, so fix the cause first. The most common causes are a wrong address (X.1.x), failed authentication (5.7.23, 5.7.26, 5.7.509) and the sending IP's reputation (5.7.1, 5.7.606 to 5.7.649). Our free SPF Checker, DMARC Checker, Blacklist Checker and Reverse DNS Lookup cover the usual suspects, and the Email Health Check runs the DNS checks in one go.
About this reference
Every entry is checked against a primary source: the RFCs at rfc-editor.org, the IANA registry, Google's Gmail SMTP errors and codes, Microsoft's Exchange Online NDR reference and Yahoo's Sender Hub. Provider messages are quoted exactly as those sources publish them. When we couldn't verify a code or message against an official source, we left it out. If a bounce you received isn't listed, the lookup still explains what its numbers mean.
Frequently asked questions
What is an SMTP error code?
It's the reply a receiving mail server gives when it can't or won't accept a message. RFC 5321 defines the three-digit reply codes, such as 550. Most servers also add an enhanced status code from RFC 3463, such as 5.7.26, followed by text that explains the reason.
What's the difference between a 4xx and a 5xx error?
A 4xx reply is temporary: the sending server keeps the message and tries again later. A 5xx reply is permanent: the message bounces back to the sender, and sending it again unchanged will fail the same way.
Where do I find the error code in a bounce message?
Look in the bounce (also called an NDR or delivery status notification) for a line that starts with three digits, often under a heading such as "Diagnostic information for administrators" or "Remote server returned". Paste the whole line into the lookup.
Why does the same code mean different things at different providers?
The standards define broad categories, and providers add their own detail. Microsoft 365, for example, uses 5.1.10 for a recipient that wasn't found, while the IANA registry defines X.1.10 as a recipient domain with a null MX. Always read the text after the code.
Do you store what I search for?
We don't save searches or pasted bounce text to any database or usage log, and search result pages are marked noindex so they never appear in search engines. As our privacy policy explains, our servers keep standard security logs of requested pages. With JavaScript on, filtering as you type happens in your browser.
Related guides
Why am I getting bounce messages for emails I didn't send?
Getting bounce messages for email you never sent? That's backscatter. Learn why it happens, how SPF, DMARC and recipient validation reduce it, and what to do.
8 min read Running email day to dayHow to read email headers
Read email headers step by step: the Received chain from the bottom up, Authentication-Results, ARC, Return-Path, Message-ID and Microsoft 365 anti-spam fields.
10 min read SPF, DKIM & DMARCGmail, Yahoo and Microsoft bulk sender requirements explained
What Gmail, Yahoo and Outlook.com require from bulk senders: SPF, DKIM, DMARC, alignment, one-click unsubscribe and spam rates, with the real dates and limits.
9 min read