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 the email messages from our mailing lists.
The phrase - on behalf of the sender is added.
This is to provide another clue of where the email really came from, so the receipent can be alert to fraudulent emails.
The email was not sent from the normal machine that sends out their emails.
Our mailing list keeps the From header of the person who sent the email to 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