A receiver-SMTP MUST implement VRFY and SHOULD implement EXPN (this requirement overrides RFC-821). However, there MAY be configuration information to disable VRFY and EXPN in a particular installation; this might even allow EXPN to be disabled for selected lists.
A new reply code is defined for the VRFY command:
|252||Cannot VRFY user (e.g., info is not local), but will take message for this user and attempt delivery.|
SMTP users and administrators make regular use of these commands for diagnosing mail delivery problems. With the increasing use of multi-level mailing list expansion (sometimes more than two levels), EXPN has been increasingly important for diagnosing inadvertent mail loops. On the other hand, some feel that EXPN represents a significant privacy, and perhaps even a security, exposure.