Your off-site Thunderbird users will need to connect to MercuryS (the SMTP server module) to send mail, that will then be delivered to local mailboxes by Mercury core and externally by MercuryC. If they connect over VPN they can send directly to port 25 on the server.
It's a bit more complicated if you want them to connect from the Internet to the SMTP server. In that case you most likely need to specify an alternative port for this, usually the submission port, 587.
The Thunderbird clients will of course need to be set up correspondingly.
/Rolf