Callbacks

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Anti-spam technique: Callbacks
Date of first use: early 2000s
Effectiveness: Low
Popularity: Low
Difficulty of implementation: Low
Where implemented: MTA
Harm: High


During an SMTP transaction, following MAIL FROM and at least one RCPT TO from the client, the SMTP server opens a connection to the host name in the MAIL FROM address 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 for that address.

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.