ESMTP The AUTH Command

The AUTH command is an ESMTP command (SMTP service extension) that is used to authenticate the client to the server. The AUTH command sends the clients username and password to the e-mail server. AUTH can be combined with some other keywords as PLAIN, LOGIN, CRAM-MD5 and DIGEST-MD5 (e.g. AUTH LOGIN) to choose an authentication mechanism. The authentication mechanism chooses how to login and which level of security that should be used. 

Below are the AUTH LOGIN  commands/mechanisms described.

S: 220 smtp.server.com Simple Mail Transfer Service Ready
C: EHLO client.example.com
S: 250-smtp.server.com Hello client.example.com
S: 250-SIZE 1000000
S: 250 AUTH LOGIN PLAIN CRAM-MD5
C: AUTH LOGIN
S: 334 VXNlcm5hbWU6
C: adlxdkej  <<<<<<<<<<<<<<<base64 converted username
S: 334 UGFzc3dvcmQ6
C: lkujsefxlj <<<<<<<<<<<<<<<base64 converted password
S: 235 2.7.0 Authentication successful

The AUTH Command