How can I stop my Mac mini's Autofill to insert incorrect Email Addresses?

Are you using Apple Mail? There is one thing that may help in Apple Mail—clearing previously used addresses.

Open Mail and click on "Window" menu to see "Previous Recipients":

Selecting that opens a window showing an editable list of prior addresses:

If the bad address shows there, delete it. I believe Auto-fill checks that list for priors. At least it is easy to check and try if the bad address is not hiding among your Contacts.

A friend's email was compromised by scammers. It was one character different from the real one and used his name. I was able to eliminate the bogus address while retaining the proper one using the method above.

Maybe… you can’t really do it for just Mac Mail alone( as far as I know. But you can turn it off globally :

Turn off Autocorrect in Apple Mail - Apple Community

Maybe in Sonoma -settings, keyboard, text input, input sources -edit- show inline predictive text OFF

that’s my best shot at it

john b

Go to AutoFill in Safari's Settings. Click the Edit button next to "AutoFill web forms: Using information from my contacts".

Correct the email address in your contact card.

Hmmm… maybe check under “Contacts”

jb

Hello, Mail>Window>Previous Recipients, highlight the ones you don't want & click the Remove from list button.