It's spam prevention. Since you can't actually receive email from an external source without screwing around with privacy settings, most mail will just bounce. Since there's no IMAP or even POP suport, the mail that does get through will be left unread.
this is about spam protection like integrating IE into Windows was about making the install easier. Seriously, this is about getting marketshare for a failing product.
I don't think it's very effective spam prevention. gmail.com and yahoo.com alone account for a large percentage of email addresses. Just add a few more to the list of domains to try, and there is a high probability you'll get something through.
It's spam prevention... from all of my friends who are the only people who should be able to see my email on facebook? What sort of signal is facebook sending about their service then?
