http://tools.ietf.org/html/rfc5322
3.4. Address Specification
Addresses occur in several message header fields to indicate senders
and recipients of messages. An address may either be an individual
mailbox, or a group of mailboxes.
address = mailbox / group
mailbox = name-addr / addr-spec
name-addr = [display-name] angle-addr
angle-addr = [CFWS] "<" addr-spec ">" [CFWS] /
obs-angle-addr
group = display-name ":" [group-list] ";" [CFWS]
display-name = phrase
mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list
address-list = (address *("," address)) / obs-addr-list
group-list = mailbox-list / CFWS / obs-group-list