Originally Posted by Geoinline
The DNS record for the domain I'm trying to send mail from.
This is how I have my DNS record setup (DNS Manager>domain.com>Records)
where 192.168.15.100 is eth0's inet addr in ifconfig.
I can send mail inside my network. Outlook works correctly (inside the network aswell)
Ok, this was a misunderstanding. If you want to use your domain also outside your network, you must use your router's public IP address for the DNS records, and forward all needed ports (e.g. 25) from your router to your ISPConfig server.