Yes, email and IM do have a lot in common -- in particular, XMPP is best visualized as an IMAP-like client/server protocol plus an SMTP-like server/server proto

Yes, email and IM do have a lot in common -- in particular, XMPP is best visualized as an IMAP-like client/server protocol plus an SMTP-like server/server protocol that both share an XML schema (ugh) for describing the messages akin to how MIME (another ugh) functions for email. As another commenter has pointed out, Delta.chat shows just how closely an email client can mimic the chat/IM interface.