All I need to do, as Cher noted, is arrow down through the autocomplete list that pops up after I've typed a character or two to limit the list, and hit the Delete key on any item I land on that I want eliminated from the autocomplete list.   They disappear immediately, and if I open another compose window and start entering the same character(s) in the To, CC, or BCC fields those addresses that I deleted remain gone.

--

Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363  

Tolerance is the positive and cordial effort to understand another's beliefs, practices, and habits without necessarily sharing or accepting them.

        ~ Joshua Liebman