When one of my users tries to send an email to a specific address, he gets this error message:
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<ouremailserver.com #5.5.0 smtp;550 sorry, mailbox unavailable (#5.5.0)>
I have tried to telnet to the destination smtp server and get the same error when I tried to manually send and email the the specific address