Jun 24, 2017

The most common authentication methods required by the mail server

Your email program may not support any of the authentication methods required by the mail server.  The most common methods are PLAIN, LOGIN, CRAM-MD5, DIGEST-MD5, and NTLM.  

For example, if you are administering a Microsoft IIS SMTP server, you must enable Basic Authentication by going here:
IIS Manager > local computer > Default SMTP Virtual Server > Properties > Access tab > Authentication > Basic Authentication > (checked)