XMPP has this neat feature that allows you to know what features are supported by a remote client, allowing you to just send social stuff to online clients that

XMPP has this neat feature that allows you to know what features are supported by a remote client, allowing you to just send social stuff to online clients that want it. Email doesn't have this, but it could be approximated by some clever use of multipart messages. It'd be even better with filters, but you can't count on things like sieve being available.