ULTIMATE GUIDE: Setting up your server for reliable email delivery. PART I: Firewall
Firewall is software (or a hardware-software appliance) that controls which connections to the server are allowed and which are blocked. In the vast majority of modern Linux server distributions, some form of firewall is available out of the box.
Reliable email delivery to the recipient depends not only on the mail server itself, but also on correct DNS records and firewall configuration. If something is wrong with those, your messages are very likely to land in the Spam folder - or not be delivered at all.
This article outlines the key steps that will allow you to achieve nearly 100% reliable delivery of messages sent from your server. In Part 1, we will walk through the possible firewall-related issues in detail; in Part 2, you will find instructions for configuring DNS records.
The information in this article applies only to mail servers running on Linux distributions. Debian 12 and Rocky Linux 8.10 with the FASTPANEL control panel are used as examples.