There is no way that I am aware of to do what you want.

But seriously, do you like errors? and wasted staff time? autofill will use the address book, so the address will almost always be correct. It takes some ham fisted manual editing to get it wrong if the address is added from the message pane. However it is very easy to add a manual space before of after the address that will cause the send to fail, and that does not consider simple typos.

Once you get the auto fill locked down are you going to pry the F9 key off the keyboard so they can not use the address bar?

What about going to the address book and simply selecting the addressee and right clicking and selecting write?

Then there is right clicking the incoming emails address and selecting Compose message to. Perhaps you need to explain why you would want folk to not use the auto fill. Then I might have an understanding of what you are trying to achieve.

We deal with patient health information. Mishandling it could result in fines that would bankrupt us. So we don't want users accidentally hitting the "a" key and having autocomplete put in Aunt Mabel's email address in the To: field. We want users to have to consciously and deliberately put names in the To: field.

They can type in an email address, then can use their address book and they can respond to an email. All fine. We don't want them to use autocomplete.

Apart from turning off autocomplete in Tools/Options/Composition/Addressing on every computer, I don't know of a way to automate this. But I don't think users are more likely to make errors with autocomplete disabled, and probably the error rate increases.

Perhaps a second line of verification is worth considering, such as using Send Later to place messages in the Outbox for confirmation of recipients' addresses, or applying a delay to the Send process when the Send button is clicked.