Messages With Large Attachments stay in mail que for long periods of time

[Meowmixjinx]

I'm a relative Linux and Zimbra newbie, but I am really liking this software as is my boss, we are planning to roll out the Zimbra server as a new mail server on the 1st of August or so, but im still having a problem, we have many people in the company that send and recive very large auto cad files, and ive noticed that when i send a hefty message from one zimbra user to another, say in the area of 130MB-170MB the message seems to get stuck in the mail cue for 25minutes and then finally shows up in the right box, its not a slow machine.

Dual Xeon 2.00Ghz
2GB Ram
3x 36GB 15kRPM in Raid5
Running Suse 10.2

Any ideas on why it takes so long to deliver?