You should be able to enter a full address without the autocomplete function
getting in the way. For instance if you are entering "firstname.lastname@example.org" as
your address but you have "email@example.com" in your address book then the
autocomplete will fill in "firstname.lastname@example.org" but as you enter the "y" the
remaining characters should disappear and you can complete the address.
You can disable this by pressing "Ctrl+," or by toggling the "Enable Address
Auto-complete" setting on the view menu of the compose window. Note: This is a
permanent change, not a temporary one (you will have to set it back manually