answer:All the ones you just named are compatible, and for the most part there is zero difference in usability in the iPhone’s mail app. All use either IMAP or POP3 for their mail protocol, so you’ll need to double check with that service to see which one they use; also, each mail system should have setup instructions on their website for iPhone users, although it’s really straightforward when you launch the iPhone’s Mail app for the first time. Otherwise, go to Settings/Mail and click the “add” button under accounts, and you’ll be able to perform the same setup process. I personally use GMail and YMail, but both function identically once you set them up. Neither sync with the iPhone’s Calendar App (to my knowledge), but I may be wrong. Now that said, those are all the free apps; the “best” user experience all-around will be either Mobile Me or Microsoft Exchange (the latter only if you work for a company that uses Exchange); but as you know Mobile Me is a paid subscription, and Exchange will only work if your company uses an Exchange server (and has set it up for IMAP).