It would be great that if disabled users cannot be included in TO and CC fields.

Some users are no longer active though they are in the past.

When you try to reply the email that is synchronized from the disabled user, he is displayed in TO and CC field still.

If you proceed the email it will end up in an error.

Also, you can still select disabled users from the search dialog box.

You need to look for a disabled user every time when this error has occurred. This is quite annoying.