SMTP
521 Host Does Not Accept Mail
The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
When You See This Code
When connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
How to Fix
Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.