Question : outlooks 2003 users disconnecting from exchange server

We have approximately 300 users connected to a MS Exchange 2003 Enterprise server to several different information stores. We are randomly getting disconnected throughout the day. Users with larger inboxes (greater than 1 GB) are more noticable.
Email is slower than usual (sending an email stays in the outbox for approximately 60 seconds)
The D Drive, located on a san, is heavily fragmented (currently 99% ) and has 50% of its available space free  (400 gb used of 800 GB total)
There are no errors in the exchange server or client machines
Is it possible that the fragmentation on the drive is causing the slowness?

Recent changes to the server
Added OMA access 2 weeks ago
Added 10 users with email storage of approximately 1.5 GB each.
The largest information store is 121 GB
There are no networking issues/ping times are normal
We notice when users remove "rules" from outlook the disconnects are less frequent
When users are in cached mode (only 50% have outlook 2003) the problem does not appear

Answer : outlooks 2003 users disconnecting from exchange server

LSI by far, as not only do HP, IBM, Sun, NetApp, Dell, Supermicro, SGI, and more slap their names on LSI controllers, but the LSI chipsets provide the back-end expanders and chipsets for the RAID vendors.   goto lsi.com, look at the controllers and select one that meets your budget.  I would go with a BBU as well, as this provides additional performance/availability benefits, especially in a degraded mode.   Also look at the SAS-2 versions that give 6Gbit/second paths.  Go with the newer Seagate SAS-2 disks as well.  Also SAS has independent read and write channels, so you can get the disks to do a lot more work before saturating things, you also get a nice upgrade path and can support more drives.