When I was a Windows SysAdmin, everything was Exchange/Outlook, the worlds most magical database (disguised as an email system). Syncing things worked, you could send/update stuff online and it just worked. Thus, everything was available offline in your .OST files on the client, even if the server vaporized.
Now that I'm out of professional IT, I've got thousands of unread emails in Gmail, and a few hundred in my last name email that I never worry about. I check email once a day, if ever.
I'd use IMAP and back up the local copy if I ever get back into needing reliable storage for my email again.