Currently we have a mail server sitting on 192.168.1.2 and we have the
following static nat

ip nat inside source static 192.168.1.2 25 (external ip) 25 extendable

This works fine, however vpn client connections can only telnet to port
25 on the mail server by telneting to the external ip port 25 which
port forwards to the mail server as per static nat.

Is there a command i could configure so the vpn client can telnet
directly onto port 25 of the mail server internally? so i can issue the
command telnet 192.168.1.2 25

The clients are using an address pool assigned by the router.

They can telnet to 192.168.1.2 110 (pop3) etc but not 192.168.1.2 25
(smtp)

Any help would be greatly appreciated as its driving me crazy slowly