Mail recipients getting mixed up

[kerm1t]

My plesk server acts as a smtp relay for another server. The other server is Windows and runs a handful of websites which send out email from php code (php.ini is set to use the plesk machine as the smtp server).

It seems that when the Windows server sends several mails close together, the recipients become mixed up. As an example, the Windows machine sent an email to pete@xxx from [email protected]; but a duplicate copy also went to webmaster@xxx. Here's how it looked in the logs:

Feb 28 17:42:28 xxx qmail-queue-handlers[2960]: [email protected]
Feb 28 17:42:28 xxx qmail-queue-handlers[2960]: to=pete@xxx
Feb 28 17:42:28 xxx qmail-queue-handlers[2960]: to=webmaster@xxx
...
Feb 28 17:42:28 xxx qmail: 1204220548.217336 starting delivery 1715: msg 67141436 to local 108-pete@xxx
Feb 28 17:42:28 xxx qmail: 1204220548.217525 starting delivery 1716: msg 67141436 to local 108-accounts@xxx

I double checked that my php code wasn't cc'ing webmaster@ in, and I also watched the email hit the server with tcpdump - the message was only addressed to pete@xxx.

However, the Win server did also send an email to webmaster@ within the space of a couple of seconds. So somehow the addresses seem to be getting muddled up.

Any thoughts please?

 

[Hedgren]

Isn't webmaster@ is an address of the Plesk administrator? BTW, the maillog is used to mix all transactions, so it is not necessarily that a copy of the email to pete@xxx was sent to webmaster@. 2 things could be checked:
- maybe there was another email sent directly to webmaster@ ?
- pete@, he received the message, does he see webmaster@ as CC in message headers?