I want a high-level test that ensures that both incoming and outgoing email services are operating normally. I've achieved this by writing a python script which does this:

1. Send a message with a unique hash through my mail server to [email protected]
2. The gmail account is configured to auto-reply back to the same address and then delete the message.
3. My script polls IMAP until it finds an email with the correct hash sitting in its inbox, or times out.
4. It reports the elapsed time to my monitoring software (Zabbix)

My question: Is gmail the best third-party to use? Should I add a couple others as well such as hotmail and yahoo? Is there anyone more official that will auto-reply to these sorts of "mail pings"? Any other recommendations for this type of test?