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.
