Detecting an SMTP provider?

I'm curious if there's an "easy" way to detect the SMTP provider a domain uses. I figured one could just inspect the domain's MX records but a lot of domains will relay/proxy SMTP traffic to their SMTP provider via a gateway. The MX record will only show the gateway hostname but not the actual SMTP provider.

Any thoughts on how to best detect the "true" SMTP provider?

Thanks!

E-Mail clients like Thunderbird and Outlook just guess different domains until they are able to connect successfully (or not). There is no protocol in common use that documents the SMTP provider for a particular domain.

Thanks for the reply. Is there any other way to determine who the "final" SMTP provider may be? Just started looking through RFC 2821 :P

If by "final" you mean the SMTP server that receives mail addressed to a domain, then you can look them up in the domain's MX records.

This may or may not be the same server that is used to send mail from accounts on that domain (which is what I thought you meant originally), and it may or may not be the same server from which the domain's users retrieve their mail. It's certainly possible that the SMTP server specified in the domain's MX record simply routes the mail to another mail server, but there is no way for you to detect that from the outside.

Also what do you mean by "provider"? How is the MX record hostname not the "provider"?