I had a reader email me and ask if it was possible to remove the From yourusername@gmail.com on behalf of customaddress@mydomain.com.
When I researched this issue to see if it was possible to change the way the “from” address is displayed, I came across numerous discussions on the “behalf of” component. I agree also that it is not the perfect way to display your email address.
It does appear to be an issue that cannot be changed within Gmail. In summary it appears to be an issue with the way Microsoft Outlook displays the “from” field.
..when you’re sending with a different ‘From:’ address, your Gmail address will still be included in your email header’s sender field, to help prevent your mail from being marked as spam. Most email clients don’t display the sender field, though some versions of Microsoft Outlook may display “From yourusername@gmail.com on behalf of customaddress@mydomain.com.”
Source: Google