What's the deal with using a port number in an e-mail address? I can't imagine many systems supporting that. Anyone got any more info on it that doesn't involve me digging through 101 pages of RFCs? :-)

I don't think that's valid. I think they misread the RFC. First of all, an IP address (address-literal) is supposed to be enclosed in [square brackets]. The colon is only allowed to designate address family, as in "foo@[IPv6:XXXXXX]".