Overview

You may want to know if ListManager has a way to add a custom email header to a message received by LM.

Solution

You can add your own custom header to the message which is received to List manager UI. It can be inserted by navigating to Utilities > List Settings > Email Submitted Content > Email header and select the Append to SMTP header tab.
Please refer to the below documentation for more information on this setting and how to correctly use it:

• http://lunar.lyris.com/help/lm_help/12.0/Content/submittedemailheaders.html