My protocol of choice when it comes to e-mail is IMAP. Its reliable, fast, and well tested.
Unfortunately, Microsoft's Outlook products have not gotten along well with IMAP in my experience, and many of my clients use Outlook.
Recently an automatic upgrade to Outlook 2003 caused a user's credentials to stop working while at the same time giving an error: "0x800CCC17 - Unable to Download Folder". Then that same week another user's Outlook stopped displaying new mail. For no apparent reason.
The first issue was fixed by reverting the Outlook update, and the second was fixed by recreating the user's Outlook account.
Thankfully another user who was experiencing similar difficulty with Outlook and IMAP was willing to try Thunderbird. So far, so good. I'm confident he will be glad he switched.