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Add a field in SMTP VRFY for "IP addresses allowed to submit that command" and then when it sees a request for that command it would check to see if the connecting IP is on that list before it proceeds with normal execution of that command. SmarterMail already checks whether the command is enabled or not before executing it, we're just looking to add an access-control list for an additional level of security.
This would make it much easier for an antiv-virus/anti-spam gateway device to securely connect to the SmarterMail server for a user account/alias sync. We use ModusGate in-front of the SmarterMail Server. We need SmarterMail to be outside the firewall so clients can attach for IMAP/SMTP through their email clients and we allow Webmail access.