I followed this guide:
All incoming mail worked perfect.
I could send mail internally.
When trying to send mail externally, i would get rejected from other mail servers.
What ended up being the problem is my IP was on a block list here:
under the PBL (policy block list).
Reason being i am testing building a mail server on a dynamic address at home.
upon requested my IP was removed and about 30 min later things started to work.
Reason i thought to mention this here is that this is the only issue i had with the install and saw no reference to it anywhere. I hope this can possibly help you too.