I have searched for an answer to this a number of times for a number of different, recent versions of Apple Mail without success. Hopefully one of the wizards on TidBITS has the answer.
When navigating here:
Apple Mail/Preferences/Accounts/anyaccount/Server Settings/Outgoing Mail Account:/drop down menu
I have a long list of outdated, unused or otherwise incorrect, smtp server entries. My goal is to clear that list to the default of zero entries. I was optimistic a plist might contain that information but if there is one I have been unable to locate it.
I am repeatedly experiencing outgoing smtp errors and believe this step may well be the solution to resolving it.