Every email system I've ever used, wants to be the only email system I use. And I won't be having that.
I want to use various email systems, and I want to collect them all into one place, using the IMAP protocol. So I have AOL accounts, gmail accounts, yahoo accounts and the various email accounts I've set up on my own servers. I use IMAP to collect all those emails into one place, and to do this I use fetchmail. And once collected in one place, I can use my homebrew despammer, and my homebrew malware-avoider. Which, by the way, is actually better than anything you can get commercially - maybe I should have gone into business on this ...
Some of them really do try very hard to stop me doing this; facebook, for example, seem to make it almost impossible. There are rumours on the web that you can use their API to implement IMAP, but I'm not going to learn their API just so that I can collect my facebook email. I can't stop facebook from creating an email address for me, but I don't actually read anything that I get there.
I recently had a bit of a wrestle with gmail. Part of that was my own fault; I changed my gmail password and forgot to tell fetchmail the new password. But even after I'd done that, there was a problem.
Gmail thinks that IMAP access from fetchmail isn't as secure as it would like it to be. So to make it work, I had to tell gmail to accept connections from "potentially insecure apps". I've never heard fetchmail being called an app before. So I did that, and now it works.
So you can email me on all the email addresses I've ever had, going right back to AOL (but not the ibmpcug.co.uk one, that's dead now). But don't email me on my facebook email address, I won't see it.