SMTP authentication not working

[Andy_N]

I have a server "example.com" with a mail account "myself" that redirects on a gmail one (myself@example.com -> .....@gmail.com).

I've set SMTP authentication (within plesk) and it seems to work.

ANYWAY: I receive much spam stating "strangeaccount@blahblah.ext via example.com" as sender, where "example.com" is MY OWN SERVER (while the mail account changes often)! So it seems someone is sending mail thru my own server, but I cannot work out how to check / protect it (and the point is that my IP could be misunderstood as being a spammer!)

In /usr/local/psa/var/log/maillog, for example, I have:
Feb 2 15:54:28 {MYOWNSERVERNAME} postfix/smtpd[22724]: connect from unknown[190.223.53.198]
Feb 2 15:54:29 {MYOWNSERVERNAME} postfix/smtpd[22724]: E6D67239E: client=unknown[190.223.53.198]
Feb 2 15:54:31 {MYOWNSERVERNAME} postfix/smtpd[22724]: disconnect from unknown[190.223.53.198]

Where the IP seems a known spamming server... and I've received a mail with that ID (E6D67239E).

What can I check more? Which files/logs?

Thankx!!

 

[Andy_N]

Is there any way to check how mail are being sent from my server? Specifically which user are logging in, so to check if there's any "hole" somewhere... I state that there are "n" users, with a strong password set.