Zimbra sends email twice to one specific provider
We are facing some strange activities in Zimbra since a couple of weeks. First of all, i'd like to inform we are still using version 5.0.18 Open Source (yes, i know Zimbra 6 is already out since months now, and we have a plan to migrate to it, but due to our customizations it's still not possible).
The problem is very simple: if i send an email to a specific provider (GMX, a very common mail provider here in Germany), the GMX user becomes two copies. We already proofed that Zimbra really sends the email twice in our logs, but we can't understand why. Between the two sendings, the log shows no error or anything special.
The strangest is that this Problem only happens with GMX, until now we still didn't experience it by any other provider.
So here comes the questions: is it a known bug? Is there any explanation for this behaviour? Or could someone at least point us to any direction on whatelse to check or test?
I must add that this error doesn't happen all the time, but it shows up really often.
Any help or tip will be greatly appreciated.