This add-on is available in all AcyMailing versions.
If you enable this add-on you can add some custom email headers, that will be added to all sent emails. These headers need to be entered in the add-on's options.
Email headers are invisible information that mail clients use. It contains the email address that sent the email, the receiver, the authentication verification results etc... In Gmail you can see them like this:
It will look like this:
For each custom header you want to add, you need to enter its name and its value.