550 5.1.1: User unknown / mailbox does not exist

AnySMTP

The error usually looks like this:

550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table

What it means

The recipient’s server says the email address doesn’t exist.

Common causes

  • A typo in the address, or the mailbox has been deleted.

How to fix it

  1. Check the spelling of the recipient’s address, or confirm it with them another way.

Tools to help diagnose it

Email server settings finder

Find the IMAP, POP, and SMTP server names, ports and encryption settings for any email address

SPF, DKIM, and DMARC checker

Check your domain’s email authentication DNS records, count SPF lookups and diagnose deliverability issues

A free tool provided by Auth-Email

Auth-Email makes your Gmail, Outlook, or other OAuth 2.0 email account available to any IMAP, POP3, or SMTP email client — no app passwords or special configuration needed.

Read the docsTry Auth-Email →

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