Change of domain name

We recently changed our domain name from @abc.com to @def.com
And we host our own domain names in house.
The default email address for the mailboxes have been changed to @def.com

However mail to user@def.com does not go through unless the email addresses in the user mailbox properties contains both user@abc.com and user@def.com.
Can anyone explain why this is happening.

Thank you