aim.com domain has no SRV records for
_xmpp-server._tcp at all, which means it doesn't support XMPP federation.
aol.com domain does have SRV records for server-to-server XMPP communications:
$ host -t SRV _xmpp-server._tcp.aol.com
_xmpp-server._tcp.aol.com has SRV record 0 1 5269 xmpp.gxmpp.oscar.aol.com.
And according to a post on the pidgin-devel list (dated May 2011),
AIM users can IM "whoever at gmail.com." The first IM will fail to go
through and the Google Talk user will get an authorization request.
After the auth request is accepted, messages from the AIM user to the
Google Talk user go through. And Google Talk users can IM
"screenname at aol.com" or "email(domain.com)@aol.com"
grawity(aim.com)@aol.com were recognized by Gmail for me, and AIM similarly refused to send anything to
firstname.lastname@example.org. I'm not sure if this is related to me having switched to Hangouts, or if the federation was just switched off.