I suspect that whatever process that verifies passwords for smtp connections is different or performs it differently than the process that verifies passwords for pop3. For example: 220 domain.com ESMTP EHLO mycomputername 250-domain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN 250-STARTTLS 250-PIPELINING 250 8BITMIME AUTH LOGIN 334 VXNlcm5hbWU6 (encoded "email@example.com") 334 UGFzc3dvcmQ6 (encoded "123456") 535 auth failure Does not work, while: +OK Hello there. USER firstname.lastname@example.org +OK Password required. PASS 123456 +OK logged in. QUIT +OK Bye-bye. Works fine. Excluding an open relay, of course, the only way I can figure out how to enable outgoing messages is to configure in Plesk Server->Mail to authenticate via POP3 and then set the outlook settings to login to the incoming server before sending outgoing messages. It should be able to authenticate via smtp, but it doesn't. Why does smtp authentication not work at all? Is this a known issue or is it just me?