It's possible to use Gmail as a client in combination with SMTPeter. To send email from your domain with SMTPeter, go to the 'Accounts and Import' tab and click 'edit info' (or 'add account' if you don't have an account set up yet).
Fill in your screen name and the e-mail address you want to use with SMTPeter. Click 'Next step'.
In the next screen, fill in your SMTP credentials and SMTPeter's domain name, mail.smtpeter.com. You also have to select the port number you wish to use.
After this, all you have to do is fill in the verification code that was sent to your address by Google and you're all set up to start sending email using SMTPeter.