In Active Directory Users and Computers there is an Email Addresses tab that lists email addresses for the user.  One of these (for each protocol) is selected as the Primary.  The primary address will be the one that a user's outgoing email appears to come from.  But there is also a General tab where one can enter an email address for the user. This also causes that user's outgoing email to appear to come from that address.  Normally, changing one also changes the other.  But if you are running Active Directory Users and Computers from a machine that does not have the Exchange Server tools installed, the Email Addresses tab is not there and changing the email address on the General tab will not change the primary.  Actually, anything can be entered in that field on the General tab, even invalid domains.  So it is recommend to always make user account changes on the Exchange server, or at least on a system that has the Exchange server tools installed.  And use the Email Addresses tab to change user's email addresses.

Some email servers perform some verification on the sender's email address and may reject it if the domain is invalid.  Nothing can send a bounce message either because the email address does not work, so the sender will not know the email was not delivered.