Frequently Asked Questions - Verification process

It's simply not possible to guarantee that. Some mail (SMTP) servers, in fact, do not reject addresses of unknown users for a variety of reasons, including:

• They're simply accepting emails to pass on to another server and thus have no idea what users are valid on the destination server. (This is often the case for secondary MX servers for a domain.)
• The server has been configured to forward all unknown user email to an administrator.
• Administrators have configured the server to accept any username to prevent hackers and spammers from guessing usernames and using SMTP to check their guesses.