Fellow MacBUer and former Entourage lead, Omar Shahine compares various mail clients on Mac, PC, and Linux and their support for IMAP.  Not surprising, given that Omar and Mike Fullerton led the design of Entourage's IMAP functionality, Entourage scores the highest among the group.  Dennis, Entourage user and well-informed commenter, noted that Entourage lacks Format-Flowed.