I don't see why anyone needs to pay for a mail client these days..
My thoughts exactly..
Before actually trying out Postbox.
It really is about twice as good as Apple Mail.
Especially with gmail, it actually works.
It makes my work so much less frustrating. And I'm happy to pay a small sum for apps that makes my life easier or let me do things more fast.
I'm a very happy customer indeed.
Can't say anything about .me tho, sorry.
Still hard to see why it wouldn't work. Check their support, I'm sure there is some discussion about it(?)
EDIT: (This might be old information so there might have been some progress, but)
"If you use MobileMe, Postbox doesn’t support MobileMe aliases natively. Microsoft Entourage 2008 has a similar problem. The workaround is to create separate accounts for each alias (which just adds to the visual clutter)."
So Aliases are apparently "secondary" mailboxes of some sort..?
Anyway, if so, in Postbox you can have "groups" of mailboxes.
Like have 3 accounts in a group "Work" and 2 accounts in a group "Personal" and in your case X accounts in a group "MobileMe". So you basically see only 1 mailbox for each group even tho they might contain several mailboxes/accounts (naturally you can browse each individually also).
Anyway, try the demo if it works for your needs. I like it a lot anyway and it works for my needs.
Good Luck!