When you send email from an application, most SMTP servers require authentication. Your application needs to supply a valid username and password to be able to connect to the server. This prevents strangers to use your SMTP server and send millions of spam emails.
When your application does not supply the correct credentials to connect to the SMTP server, instead of a descriptive message you can get this:
Mailbox unavailable. The server response was: <SEND_TO@SEND_TO.com> No such user here
The error message complains about the addressee, the email address you are sending the email to, instead of the username and password you try to use to access your SMTP server.
Make sure you are using the correct username and password to connect to the SMTP server.