Settings → Misc Settings → External Fax

The External Fax tab is for use with some external fax server applications which work by collecting mail from a POP3 mailbox.

If a Fax Server Mailbox is selected, then VPOP3 will automatically send any locally sent mail to that mailbox if the recipient is local, but doesn't match another local address (a user, list, mapping etc), and if the recipient address is a telephone number

For instance, a message to 01234123456@mycompany.com will be sent to the Fax Server Mailbox if there is not a local user, mapping or list called 01234123456.

If the Allow External Access to Fax Server Mailbox box is checked, then incoming mail which matches these rules will also be sent to the Fax Server Mailbox, otherwise only locally sent mail will be allowed to this mailbox.