The software described in this documentation is either in Extended Support or Sustaining Support. See https://www.oracle.com/us/support/library/enterprise-linux-support-policies-069172.pdf for more information.
Oracle recommends that you upgrade the software described by this documentation as soon as possible.
Several different network protocols are required to deliver email messages. These protocols work together to allow different systems, often running different operating systems and different email programs, to send, transfer, and receive email.