I'm using 1 POSTFIX as MTA and 1 Exchange2003SP1 as backend.
In normal situation, everything is okay and the delivery speed of smtp mail to outside is quite fast.(less than a min.)
However if someone(outlook user in inside) send the message to non-existent domain name with typo like "domain.con" (should be ".com")
of cource this message will be stayed on the Exchange smtp queue and reply back with NDR to the sender, but during this period(stay in the queue),
other email also start to stay in the queue. Normal email process delays few mins because of the wrong email.
I've tested this case in the two quite different environment and got a same result.
If you are Exchange admins, please let me know if you can duplicate the issue.
I'd like to resolve this issue. Only one worng email shouldn't be affected to others, I believe.