Callbacks: Difference between revisions

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SMTP server connects to SMTP client on port 25 during transaction.
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.


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.
 
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.

Revision as of 17:37, 6 April 2008

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.