Summary
The Mailserver on this host answers to VRFY and/or EXPN requests.
VRFY and EXPN ask the server for information about an address. They are inherently unusable through firewalls, gateways, mail exchangers for part-time hosts, etc. OVS suggests that, if you really want to publish this type of information, you use a mechanism that legitimate users actually know about, such as Finger or HTTP.
Solution
Disable VRFY and/or EXPN on your Mailserver.
For postfix add 'disable_vrfy_command=yes' in 'main.cf'.
For Sendmail add the option 'O PrivacyOptions=goaway'.
References
Updated on 2015-03-25
