When sending email out from an email domain forwarded to gmail (Send mail as), gmail appends the orginal (gmail) email into the header files of the email (it could be your private gmail account) along with the reply email.
In order to hide/mask that email use this workaround.
- Goto Mail Settings.
- Click “edit info” for Alias.
- Click “Next Step” (to skip the Names screen).
- Choose the “Send through xxx.com SMTP servers (recommended for professional domains…”
- For SMTP Server, type “smtp.gmail.com” and choose port 465.
- Type your GMAIL username and your GMAIL password in the respective fields
- Check the box “Always use a secure connection”
- Save Changes and you’re done!
Copied with permission from danieltsou.com.