Install smtpprox-loopprevent by entering the following commands in the terminal:
sudo apt update sudo apt install smtpprox-loopprevent
Description:
Transparent SMTP proxy to prevent mail forwarding loops
smtpprox-loopprevent is a transparent SMTP proxy which rejects a message if any of the recipient addresses match a Delivered-To header. . It was written to be used as a Postfix before-queue filter.
Homepage: https://github.com/jnorell/smtpprox-loopprevent
Version: 0.1-1
Section: universe/mail