Hey, I know why this isn't working and it has nothing to do with the plugin. This has to do with the fact that you're using the Gmail server as your outgoing mailserver.
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.
I ran into this issue on several occasions before, both on the free Gmail service and the paid Google Apps for Work service and it doesn't seem like there's a way around this.
I solved the issue by simply using a different outgoing mailserver which does not have this restriction.
Even though this was posted three months ago, I hope it will be useful to someone. It may just prevent you from pulling your hair out trying to figure out why it isn't working as expected, even though all settings are correct.
- Rinck