Transport
Once email message headers have been parsed and recipient
addresses identified, the message must actually be delivered
across the Internet. This is done using DNS and the
SMTP protocol. Thus email sent to
Connected@FreeSoft.org will
trigger a DNS lookup on FreeSoft.org. The lookup
will be first for an MX (mail exchanger) record, then a
normal A (address) lookup if this fails. MX records
contain a preference value (to chose among multiple MX records
for a particular domain), and the DNS name of the host
receiving mail for this domain. This procedure
is documented in RFC 974.
Messages for a particular user will typically be
placed in a mailbox, which then accessed by the user
using protocols such as POP and IMAP.
