Budrys,
I need more infomations for that.
Where are your users? Local in your company? Will you send mails from locally users to external users?
The problem is not the mailsystem the problem is your understanding how all this works together.
I'll try to help you but you need to know what you exactly want.
For a correct setup of any mailsystem the DNS is mandantory If you are unable to set the DNS correct your can't use the mailsystem for external use.
And you must know your network infrastructure.
- Are firewalls between you and the internet?
- Will your IP be natted thouth the internet?
- How shuld the incoming mails routed to?
Regards,
xabbu