Hi,
Thank you for contacting us!
The fact that you're using the Gmail server as your outgoing mail server.
As an anti-spam measure, Gmail will not allow you to modify the senders' email address for messages; even if you submit the message to Google's servers with the proper "From" header, it will simply rewrite the from header and replace any email address with the Gmail account email address you are connecting to.
Thanks & Regards
Support Team
Tech Banker