How to turn off "on behalf of" in Gmail
To fix the issue, you just have to set up your alternate account to use its own mailserver instead of Google's. Go to "accounts" in your Gmail settings, and under "Send Mail As," you should see your alternate addresses. Click "edit info" and "next step," and you should find the option to send using your own SMTP server.
You'll just need to enter the username and password, and you're good to go. Goodbye, "on behalf of!"