Services → SMTP Server → VRFY/EXPN Tab

VRFY and EXPN are SMTP commands which allow another computer to verify email addresses without sending mail to them. See RFC 5321 for technical details.

Usually, these commands should be disabled for security purposes, but some other software may be able to use these commands for various reasons. In this case, you can enable them, but restrict access to the IP address(es) you trust.

This option enables the VRFY and EXPN commands. Normally this would be turned off, unless you have some other software which requires them.

If the VRFY and EXPN commands are enabled, then you can set 'Access Restrictions' for these commands in this section. IP addresses and networks which are allowed here will be able to access the VRFY/EXPN commands, other IP addresses will receive an error if they try to use the commands.