As already noted in the comments, the last part of the User logon name is not an email domain.
The logon name, or User Principal Name, is made up of a user part (usually the same as the users SamAccountName) and a UPN suffix.
UPN suffixes are defined in Active Directory at the forest level, and you can add new UPN suffixes pretty easily, given that you have an Enterprise Administrator account in the forest:
- Open Active Directory Domains and Trust
- Right-click the Active Directory Domains and Trusts label in the tree view and select "Properties"
- Go to the "UPN Suffixes" tab
- Add a new suffix
Even though it's not email domains, all UPN suffixes should adhere to regular DNS naming conventions.
A number of AD integrated middleware applications like Lync 2010/2013 can automatically add user principal names as SIP addresses, in which case you might want to add a UPN suffix matching the public SIP domain.
In Exchange however, recipient addresses are easily managed with Email Address Policies. Unless the local domain UPN suffix is causing problems, just leave it be :-)