Now the mail is incoming to server
mail server             monowall                                    cisco
adsl
192.168.0.1----192.168.0.254(sis0) / 10.0.0.1(sis1) -- >
10.0.0.254 --internet

We need this nat rule for incoming mail:

Firewall:Nat -> Proto (tcp) - Ext port range (25 smtp) - nat ip
(192.168.0.1) - int port range (25 smtp)