In the past month I've seen this issue arise where Outlook freezes sporadically when sending, replying, and even printing for some. I've been seeing some other questions on EE about freezing which has lead me to do things like make sure the Exchange Servers and clients are both up to date with the latest updates, disable add-ins in Outlook. Initially I was looking at the Exchange server to make sure there was no pressure on the resources. I also tried office repairs, removing & recreating outlook profiles.
I'm not sure what to look at in Exchange but from reading all the other posts and answers I didn't see any indicators that Exchange itself would be the issue. To throw this in there also, we currently have two domains in this building and each has their separate Exchange environment and users on both domains are seeing the issue. One of the servers/environments only contains 20 users and the other 500+.
Domain 1 - 20 users
Outlook 2010 on Windows 7
1 CAS/HUB server - HP Proliant DL360 G7 (6GB RAM, 136GB local drive space)
2 Mailbox servers - HP Proliant DL360 G5's (16GB RAM, 136GB local, 500 ISCSI space)
Domain 2 - 500+
Outlook 2010 and some 2007 with mostly Windows 7 and a few XP
2 CAS servers - HyperV VM (16GB RAM, ISCSI storage)
2 HUB/MBX - HyperV VM (16GB RAM, ISCSI storage)