How make Mac OS mail Connection Doctor forget older accounts I have created a number of "internet" accounts for mail in Mac OS, but for various reasons they didn't work, and I deleted them from the listings in the preferences, and of course they therefore don't appear in the Mac OS Mail app either. In fact, there are now zero accounts registered with mail abilities and zero appearing in my Mail app.

BUT, if I go to Mail, and choose Connection Doctor, it still scans those old email accounts to see if they can connect or not! 

So it is clearly "retaining" those old accounts... and their credentials!! (because they are actually giving green lights for successful connection!)  ...

How do I get it to forget those old accounts? Cleanse that oddly cached account info?

Thanks!
Apple's Mail app stores the settings for SMTP servers apart from the settings for IMAP/POP3 servers. In my experience, this often leads to the fact that after deleting mail accounts, the SMTP servers are still visible in the connection doctor although all other data regarding a certain internet/email account have been removed.

In case the connection doctor shows you only the SMTP accounts of the already deleted mail accounts, please proceed as follows:


Open Mail > Settings > Accounts > SMTP-Server > Edit SMTP-Server list
In the dialog which opens: Select entries in the server list
Remove items by clicking on the minus-symbol below the list


In case the connection doctor also shows you the IMAP or POP3 account information of the  already deleted mail accounts, proceed as follows:


Quit Mail
Re-add the accounts using System Preferences > Internet Accounts
Open Mail
Quit Mail
Remove the accounts using System Preferences > Internet Accounts