GMail SMTP server adds a Sender: header, which Outlook [idiotically] displays as "From: <Sender: Header> on behalf of <From: Header>":
My Sprint cellular modem does the exact same thing. If one of my email recipients forwards my email to someone, the person receiving that email will see <
[email protected] on behalf of N965VJ [
[email protected]] >.
When I first got an Aircard I didn’t realize that email replies from a person that received the forwarded email went down a black hole. Level 2 tech support at Sprint could not figure out why that “on behalf of” stuff was going on or how to stop it. I gave in and created a POP 3 account in Outlook to handle this.