I prefer an email client that keeps multiple user accounts truly separated, not forcing me to write rules to move incoming mail to different folders. Therefore I still prefer PMMail/2 (an old OS/2 POP3 client) to all others. Thunderbird is at least similar but isn't as slick or intuitive as PMMail/2 was. But nowadays I do my email via a web interface and not by any of the providers listed in this poll.

I use Mail with multiple POP and IMAP accounts, and messages are delivered in to separate inboxes.

Actually, by default it looks like they are all coming in to the same "Inbox" but if you click the little triangle next to the inbox it will open to show you inboxes for each of your accounts. The same goes for the "Drafts" "Sent" and "Trash" Folders.

I actually like the fact that I can then set up filters on any incoming mail regardless of source, and have the messages moved from one POP or IMAP account to another IMAP Account / Folder.

At the moment I have 10 accounts set up in my Mail and it is handling them very neatly and efficiently.