I'll admit that I'm a little vague about what Apple is showing in that picture. For me, the list shown there contains the list of accounts I have for receiving email. Maybe Apple knows how to deduce the smtp address from the imap address for those configured accounts. (Come to think of it, Apple deduced the iMap server address from my email address, since I don't remember ever entering it.)
If I select "Edit SMTP Server List", I see a few other SMTP servers which are not implicitly associated with any configured account. I was then able to delete one of those successfully.
Be warned that Apple has a bug here. When I deleted an entry from that list, all of my configured email accounts lost their implicitly assigned SMTP servers. So, they all had to be reassigned. It wasn't a big deal even with the 9 email accounts I have.