Primary and alternative mail addresses require a valid domain, that is already registered. If a new address has no valid domain, the user should get an information about the available domains in the "error"-popup.
This issue has been filed against UCS 3. UCS 3 is out of the normal maintenance and many UCS components have vastly changed in UCS 4. If this issue is still valid, please change the version to a newer UCS version otherwise this issue will be automatically closed in the next weeks.
*** This bug has been marked as a duplicate of bug 54467 ***