I’we been lookng around for a guide/tutorial on how to setup a secure mailserver on the BBB (running Ubuntu).
Has anyone successfully managed to do this? And do anyone know about a guide for this?


Standard PC / x86 instructions should apply here. Assuming packages for set of instructions followed are available. This applies for most (if not all ) similar situations.

To clarify, “Linux is Linux”. So any modern set of instructions that you can find on the web should work. If in doubt that a package for a given set of instructions exists . . . apt-cache search