Disable mail account creation, but allow creation of FTP accounts

I am wondering if there is a way to disable the mail features for an account, but still allow domain users to create users on the system for FTP access? We have a lot of customers with mail hosted elsewhere for their domain, but they want the ability to have a couple FTP accounts in their domain. Currently, I have mail enabled and it just doesn't work, but I have to go through after creating each site and pull out the virtual entries for postfix so that the system doesn't error out thinking it is authoritative as the mail host for their domain.

I figured out I can disable the addition to the /etc/postfix/virtual.db file by editing '''virtual-server-lib.pl''' and '''feature-mail.pl''' and commenting out references to the &create_replace_mapping function, which basically solves the problem with virtual entries, but it seems there should be a cleaner way to handle this. What if later I do want to have some real mail users?