In OSX Mail, how do you type an address without completion/correction?
After brute-force experimentation, I found the solution is the Escape key.  Type either "Esc, comma", "Esc, Tab", or Esc and then click on the body or another header.

Esc must come after the very last character.  (I'd prefer a way to mute autocomplete until I start on a new address, because the flashing makes it hard to type.)

Other natural-seeming solutions do not work.  E.g., if I type "delete", Mail hides the autocompletion, appearing to respect the typing.  If you then type "Tab" or "comma", or click elsewhere, though, even without another character typed, it replaces the adress with the first match in its list, and you must delete and start over.