What is causing the "on behalf of" text in the From header?
Some email clients, notably Outlook and Gmail, will change the From: header of email messages from our mailing lists.
When the email is not sent from the normal server for that sender, the phrase "on behalf of" is added.
This phrase provides a clue of where the email really came from, so the recipient can be alerted to potentially fraudulent emails.
Our mailing list keeps the From header of the person who sent the email to the mailing list
However, the message itself is sent from our machines to all members of the mailing list.
Those email clients are altering the From: header to include the mailing list that sent the message
In this case, firstname.lastname@example.org