http://www.aqua-mail.com/download/AquaMail-market-1.3.0-pre1.apk+ Microsoft Exchange (EWS).
Automatic setup for Office 365 and Exchange Online.
So far, just mail sync. Server side contact lookup should come soon, full contact and calendar sync to be added somewhat later.
Mail sync: read/unread/flagged state sync both ways, access to folders, moving messages between folders, all three ways of deleting messages, sending (of course), attachment and inline preload, server side text search.
Meeting invites right now come through as regular text messages (not invites proper). I'm hoping to fix this by Monday.
My test environment is Exchange Online, just one account, so I'm sure there will be compatibility issues. Please report them by email or here, I'm going to subscribe to this thread.
Exchange 2007SP1 or higher is required (released November 2007). The original Exchange 2007 version without any service packs is not supported and won't be.
Kerio is not supported and won't be, based on what I've ran into.