I receive a lot of email messages from a third party service, so every email comes with a from firstname.lastname@example.org via third party. How do I get the email and send my automated response to the original sender, but not the third party service, because my canned responses are sending to the third party service?
The third party you're talking about is usually called an Email Service Provider (ESP). These companies offer dedicated bulk email services either for marketing or high-volume system messages (account creation, order confirmation, etc).
Unfortunately it's a bit confusing to identify the "Sender" address as that can mean different things, such as the:
Things get even more difficult as many ESPs control all three of these addresses. They may or may not even forward replies to any of these back to their client, depending on the contract they have in place. As every ESP operates differently (and there are loads out there) there's no way to determine the true origin with any consistency, and certainly no easy way to bypass the ESP.
Term definitions from ReturnPath blog post.