Callbacks

SMTP server connects to SMTP client on port 25 during transaction. Usually, following MAIL FROM and at least one RCPT TO, the SMTP server opens a connection to the SMTP client system on port 25 and uses the RCPT TO value in a MAIL FROM commands, and the MAIL FROM value in a RCPT TO command, in an attempt to see if the client system is an SMTP server and accepts mail.

This is a deceptively attractive approach, since it appears to validate the client system as a proper server. However, it quickly runs into numerous problems. The most severe is that if both systems try this, they both deadlock and mail cannot be exchanged. Other problems include temporary unavailability of the mail system, sites which operate separate systems for sending and receiving mail, etc.