We have the following network structure:
<adsl modem="">–- <pfsense fw="">--- <lan>.
We would like to connect in the lan area a mail server. What is the proper way ?
Bridge mode in adsl modem and NAT in fw or NAT in adsl modem ?
Any example/suggestion or reference for the configuration ?</lan></pfsense></adsl>
Bridge the modem. Then you don't have to configure anything there as it is more or less a "dumb" mediaconverter. Then create a portforward at your pfSense (firewall>nat, portforward) for protocol SMTP (TCP 25). This will be enough to receive mail. Depending if you want to grant clients at WAN access for checking their mails you'll need POP3 or IMAP forwards as well.
In case that Bridge mode is not working what should we do ?
Then do a double nat. Set the modem in router/nat mode and search for an option called "dmz" or "expedited host" in the modems webgui. Assign the pfSense WAN IP there and everythig will be forwarded to the pfSense. The only things that don't work nice with such a config is the integrated dyndns client (as pfSense doesn't see it's real WAN IP anymore) and maybe IPSEC (unless you configure a different identifiers than "my ip address").