What are common 5xx codes and their meanings? (e.g., 550, 553, 554)

You fire off a campaign and a chunk of your list bounces back with 5xx codes. Unlike 4xx temporary failures, which tell you to try again later, a 5xx means the receiving server is done with this conversation. Permanently. No retry will help.

Here's what the most common 5xx codes actually mean and what to do with each one.

550 Mailbox not found (or policy rejection)
This is the most common hard bounce you'll see. Either the user doesn't exist at that domain, or the receiving server's policy blocked your message outright. When 550 is about a non-existent address, suppress it immediately. When it's a policy block, the full error message will usually hint at why (spam content, blocklisting, sender reputation). Fix the underlying issue before retrying from a policy-blocked domain.

551 User not local
The server doesn't handle mail for this address and isn't going to forward it. This is often a sign the address is wrong or has changed. It's worth double-checking the address format, but don't keep hammering it.

552 Message too large
Your email exceeded the recipient server's size limit. This isn't an address problem. It's a content problem. Shrink your message, strip heavy attachments, and host large files in cloud storage (like Google Drive) instead of embedding them in the email.

553 Mailbox name not allowed
The address itself is invalid or the server's policy rejects it. This can mean a typo in the local part of the address (before the @) or a strict policy on accepted sender formats. Suppress the address and check your list for format errors.

554 Transaction failed
This is the catch-all permanent failure. The accompanying message is where the real detail lives. Common causes include spam filter rejection, blocklisting, or a DMARC failure on your end. Read the full error text before deciding what to do. If it's blocklist-related, check your domain and IP reputation first.

What to actually do when you see 5xx codes

• For 550, 551, 553: suppress the address. These are recipient-level failures. Retrying hurts your sender reputation and wastes sends.
• For 552: fix your email content, not your list.
• For 554: read the full diagnostic message. If it's a sender-side issue (policy, authentication, blocklist), fix that before sending to anyone else.

A clean list makes 5xx codes far less common before you even hit send. If you're seeing a high volume of 550s after an import or a long gap between sends, it's a sign your list needs a check-up. You can run it through RME Clean to catch dead addresses before they bounce (and before they damage your reputation).