One of the less understood components of a working email system is the MX record, or mail exchanger record. The MX record is a type of resource record in the Domain Name System (DNS) that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available.
For example, when you send an email to firstname.lastname@example.org, your mail transfer agent will lookup the MX records for paubox.com. Your mail transfer agent will find mx1.paubox.com as the only MX record and will establish a connection with this server to send the email. If there are multiple MX records found, your mail transfer agent will typically follow the priority designation on each MX record.