The network owner where I have my current server is not very good at maintaining proper DNS and reverse-DNS records (!), and I consider it to be really important for mail servers to have those properly configured. In order to workaround this issue I’m now renting a Virtual Private Server (VPS) that allows me to set the reverse-DNS record myself, and that is the new SMTP server for my domains.
I checked the mail log from time to time to see that things were working properly, and after a while I noticed that a number of SMTP servers were refusing mail to me. My server had been listed in The Spamhaus Exploits Block List (XBL) for some reason! Why?
My server was actually listed in the Composite Blocking List (CBL), and it was listed because it sent a bad HELO in its SMTP conversation. After a minor wild goose chase I realized that I simply had forgotten to change myhostname in /etc/postfix/main.cf from the default! I corrected the hostname setting in postfix, verified that HELO was correct, requested unlisting from CBL and things worked fine the next day.