I'm having a problem with an ISPConfig server where it seems to restrict outgoing mails if the IP addresses originate from outside my country. I checked the Postfix settings and firewall rules, but haven't identified anything that could cause this behavior. Receiving mails works but sending does not. Any ideas where I should look?
This is not the case. If there is such a restriction, then it's not from ISPConfig or a component of the default ISPConfig setup. My guess is that not your server restricts something but other mail servers refuse receiving the email e.g. because you might not have a proper PTR record, IP is blacklisted, no DKIM signing and so on. Which exact error message do you get in your mail client and also in the mail.log
The error message from the Mail client just states that it can't reach the SMTP server. (It does work though, as soon the client is connected to any VPN in the same country as the server) Interestingly, if I search for the client's IP address in /var/log/mail I can see it connecting to IMAP but there isn't an entry for it trying to connect to postfix.
Ok, so we are not talking about an outgoing IP then, but the incoming IP. Basically your client can't connect to your server using smtp. There is no kind of country based block in the ISPConfig software stack, most likely the provider that provides the internet access for the server or the provider that provides internet access for the client are restricting you here. In case you use port 25 in your mail client for smtp, try using port 587 as many providers block port 25 for mail clients today.
Another thought, though it is obviously solved but ... E-Mail headers contain the IP / Clientsoftware of the sender. Like if you send using Outlook, it can be very well seen if not stripped. Thus I could add a filter on my system to check for that.
