I use Gnus. I've got it setup so all my incoming mail lands in Gmail. Then, Gnus checks that account via POP3. Gnus only checks mail when prompted, so anything

I use Gnus. I've got it setup so all my incoming mail lands in Gmail. Then, Gnus checks that account via POP3. Gnus only checks mail when prompted, so anything that Gnus hasn't fetched yet shows up as unread when viewed on the web or via iPhone/IMAP. Gnus is setup to select the correct SMTP server based on the contents of the message being replied to or the view that initiated compose-mail.