Extrernal mail not working

My old Freebsd server has died of old age, and I have built a new one using Freebsd 8.2. Almost everything works fine, Apache, Samba, Qpopper etc but I cannot get external mail to go out or come in.
I cannot 'telnet domain.com 25' - no connection. I can 'telnet localhost 25'. It's not my ISP blocking anything - I have had to move email to the PC on the table next to it in order continue to receive mail.

I can ping the outside world, so I imagine it's not DNS.
I can mail local users.
I can appear to email external users, but nothing gets received.

I have set up and made a hash of virtusertable, and done everything exactly as I have done in the past since v4.11, using my notes. local-host-names has the same list of names that Samba's virtual hosts have.

Can anyone help me with some tests to try, or logs to look in, or is there some known difference between 8.2 and earlier versions?

Is your firewall and any port forwarding set up properly?

I discovered what caused it last night. PING, ports etc are all happy with the router's internal IP address as the DNS server.
Sendmail is fussier, and if you don't use the external IP address of the DNS server, names don't resolve, or perhaps it gets confused when the real server replies. At least, changing that on my setup made it work.
Thanks for the help.