So I've been trying to set up a combined postfix / dovecot server for receiving and sending mails, mostly following this guide:
I've now managed to get most things working including ssl/tsl. However, I'm still not able to receive emails at all. Also I cannot send email out of the server if I do not specify a relayhost (in that case it works perfectly). I can send emails using telnet both from within the server, and from within the local network. However, I cannot access it from outside the network.
Here is where I'm confused. I can ssh into the server from outside the network, so it certainly is accessible, even on a custom ssh port. Also, I can telnet into port 25 from within the local network. And if i do an nmap from within the network, ports 25 and 465 are open. However, from outside the local network, all ports are listed as 'filtered', except for port 22 which is 'closed' and my custom ssh port which is 'open'. What does it mean for a port to be filtered, and how can I determine the reason for this, and possibly fix it? Finally, am I trying to do something which cannot be done -- is it impossible to have different email providers, such as google, etc, deliver mail directly to my server?