I'm beginning to feel like your stalker : http://forum.parallels.com/showthread.php?t=70571

We've always seen it with Outlook and Macmail where I work (which meant it predated 2009 when I started here). I've only skimmed that old thread, but it looks like poplocks causes it... Kind of makes sense if that takes priority over smtpauth with Outlook sending an auth request and Qmail thinking it doesn't need one.

With regard to spamdyke and poplocks, I'm pretty certain that its not compatible and thats the reason why we don't use spamdyke (worryingly we've started to these last few days!). This also makes sense since it hits spamdyke before it hits plesk/qmail/poplocky stuff and spamdyke will just tell the user to piss off since it knows nothing about the poplock auth.

Paul.

Yup. That's the thread.

Yes, SpamDyke and pop-before-relay are definitely not compatible. This isn't a bad thing really, as nobody should be using pop-before-relay ideally (but we are, on one system, so no SpamDyke on that one).

I've just checked our settings in plesk... am I right in thinking:
Relay options
------------
Relaying

authorization is required:

POP3 before SMTP lock time Min (UNCHECKED)
SMTP (CHECKED)

.... Means we don't allow pop before relay. In which case, that's probably not the cause of our problem with spamdyke

yes, correct unfortunately.

If you are using the ART RPMs, have you tried compiling manually (and vice versa)?

I manually compiled it from spamdyke.

You're not using RBL from within Plesk, are you? RBL in spamdyke is the way to go.