Hello Community, Thank you for posting the amazing script Perfect Server Automated ISPConfig 3 Installation on Debian 10 and Ubuntu 20.04^ I actually run into one more problem as at one point the server would not receive any emails from other ISPConfig servers (I didn't try Gmail etc.. probably should have). What has happened: I sent an email from server1.example.com on 11/5/2021 where I installed ISPConfig manually many months ago (the mailserver was and is working fine there) but the newly installed server2.example.com (using the automatic installer script) would not receive the email. I sort of forgot about it and on 16/5/2021 I received an email from [email protected]server1.example.com : Subject: Undelivered Mail Returned to Sender Code: This is the mail system at host server1.example.com. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the attached returned message. The mail system <[email protected]>: connect to server2.example.com[18.104.22.168]:25: Connection timed out Reporting-MTA: dns; server1.example.com X-Postfix-Queue-ID: 753FB3EAD5 X-Postfix-Sender: rfc822; [email protected] Arrival-Date: Tue, 11 May 2021 11:25:15 +0000 (UTC) Final-Recipient: rfc822; [email protected] Original-Recipient: rfc822;[email protected] Action: failed Status: 4.4.1 Diagnostic-Code: X-Postfix; connect to server2.example.com[22.214.171.124]:25: Connection timed out (126.96.36.199 was an actual public IPv4 address of server2.example.com) I didn't have time to deal with it on 16/5/2021 so I looked at it again today (17/5/2021) so I scanned the 188.8.131.52 IP with nmap tool and indeed the port 25 wasn't opened (as I followed the firewall recommendations within the howto^) Later I opened the TCP port 25 via the web interface (System-Firewall) even though it was not advised and I sent an email again from server1.example.com to the same server server2.example.com and somehow the email arrived, I figured that's what fixed it so I run nmap again but the port 25 wasn't opened (?!) at least according to nmap. So I closed the port again (removed it from the list of opened TCP ports via web interface) but the email would still work (?!). Can somebody explain to me what has happened or how to debug this? Could it be related to not yet fully propagated DNS records on server2.example.com? Why would I get the failure email 5 days later? Is it possible that there is some sort of weird bug within the firewall? Does the ISPConfig actually use port 25 to exchange emails between servers? Thank you for any input on this.