SMTP Error 550 - Mailbox Not Found / User Unknown

What does this error mean?

SMTP error 550 is one of the most common email errors. It indicates that the email could not be delivered because:

The recipient's email address doesn't exist
The mailbox has been disabled or deleted
Your email was rejected due to policy restrictions
The recipient's server is blocking your messages

Common Causes

person_off

Invalid Email Address

The email address has a typo or the user account was deleted.

block

Sender Blocked

Your IP or domain is blacklisted by the recipient's server.

policy

Policy Rejection

The recipient's mail server has policies that block your message.

shield

SPF/DKIM Failure

Your email authentication records are missing or misconfigured.

How to Fix

1. Verify the Email Address

Double-check the recipient's email address for typos. Try contacting them through another channel to confirm their current email.

2. Check Your Sender Reputation

Use our Blacklist Checker to see if your domain or IP is listed on any blacklists. Request removal if listed.

3. Configure SPF, DKIM, and DMARC

Ensure your domain has proper email authentication records. This helps receiving servers trust your emails.

4. Contact the Recipient's Admin

If you're certain the address is correct, the recipient's IT team may need to whitelist your domain.

Example response

550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table
550 5.7.1 Service unavailable; Client host [x.x.x.x] blocked using Spamhaus
550 Requested action not taken: mailbox unavailable

Related SMTP Errors

551 User Not Local 553 Invalid Mailbox Name 554 Transaction Failed 450 Mailbox Unavailable (Temporary)

Mailbox Not Found