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I have a web and mail server with Ubuntu 10.04, Apache 2, courier, postfix, Ispconfig 3 and multiple sites hosted.

The clients use mail from Outlook with a config like this:

@domain1.com use pop and smtp server mail.domain1.com
@domain2.com use pop and smtp server mail.domain2.com
@domain3.com use pop and smtp server mail.domain3.com

But some of then use mail.domain3.com, although their mails are @domain1.com or @domain2.com This works, since the different domains have the same IP, but domain3.com is going to dissapear.

¿How can I detect from the server who is using mail.domain3.com?

Mails logs didn't tell me and Apache logs only have web visits. I tried syslog and message logs too but nothing. Thanks in advance.

Lecquio
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    You turn it off and wait for them to call the help desk. – Michael Hampton Apr 13 '15 at 07:32
  • Increase mail logging for mail.domain3.com via postfix/courier, wait a few days, check for authentications (possible in auth.log instead of mail.*) send those users an email or change it manually. Additionally you "could" temporary(!) change your company DNS entry to have domain3.com link to for your setup correct IP, at least until you finished merging all user setup to the other 2 domains. – Dennis Nolte Apr 13 '15 at 08:31
  • Neither POP nor SMTP have the equivalent of host headers in HTTP: the client program resolves the hostname so I can't think of a situation where the particular domain name they have set up will ever be sent to your server... – Bandrami Apr 13 '15 at 09:17
  • I'll try increasing logging level.If this doesn't work, I'll call costumers, what I was trying to avoid. Thanks all. – Lecquio Apr 14 '15 at 08:29

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