Albert,

See either:

Delete an Address From the Outlook Autocomplete List

Outlook 2019/365: Clear Email Address From Auto-Complete

 
Remember that autocomplete is not looking at your contacts, but through a list that it creates over time as you send e-mail messages.  This is emphasized in both of the above articles/tutorials, so there will be many more entries associated with autocomplete than are in your contacts.

You can either delete individual entries if you know part of the address you want to delete, or you can nuke your entire autocomplete list.  But if you elect to do the latter, you will be starting over from scratch as far as it being created again as you send additional email messages, and many of the entries you have become accustomed to "just being there" will not be until you actually send a message to that address again.

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Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042  

It is better to be hated for what you are than to be loved for what you are not.

           ~ André Gide