can't get mail server to connect using name instead of ip

[confusedstingray]

Ok i have followed the howto to set up for virtual-users-domains-postfix-courier-mysql-squirrelmail-ubuntu8.04. it works internal send and receive mail internal by ip. in thunderbird i set the smtp server to the ip 192.168.x.x and it works. but if i set it to my mail server name mail2.mydomain.ca it connects and sits there doesn't ask for user name or passwrd. went to mxtoobox.com and a few other test sites and put the mail2.mydomain.ca in and it connect and give a response.
the only thing i can think of is, i'am sending out to internet(www) and the trying to come back to private lan through my router. also my mail2.xx.ca is resovling to my external ip address. tested that at the mxtoolbox.com site
as well. any suggestions would be great,
Thanks!!!

[confusedstingray]

i think i have figured out something i have 2 mail servers 1 i can control and 1 i can not, mail.mydomain.ca i can't control. mail2.mydomain.ca i can control postfix server. so if this is possible i would like to have postfix receive all email address receive email for users on the postfix server and foward the rest to other mail server is this possible ( sorta load balance ) by email addy.
thanks

[falko]

You can direct the MX records of your domains to mail2.mydomain.ca and then create transports on that server to forward certain mails.
See

man 5 transport