Looking in the customers ccm.log we saw many errors for a single user which was not the user that wasn't receiving the build notifications:
"An error occurred while processing build results for build email
notifications:
com.ibm.team.repository.common.model.impl.ChangeEventImpl@68906890
(stateId: [UUID _r-w3kNo6EeWz6f-oWwUUxw], itemId: [UUID
_r-w3iNo6EeWz6f-oWwUUxw], origin: <unset>, immutable: true) (contextId:
[UUID _7G7qr_U1EeKRx5VfiF883A], modified: 2016-02-23 08:35:29.85,
workingCopy: <unset>) (predecessor: <unset>) (eventState: [UUID
_r-LBp9o6EeWz6f-oWwUUxw], eventOwnerNS: com.ibm.team.build, eventTime:
2016-02-23 08:35:30.85, eventCategory:
com.ibm.team.build.category.BuildResultChanged, eventTitle: Failed: test
QA - Server 10.0.7_20160223-0829, eventDescription: <p>Build: test QA -
Server<br>Status: Failed<br>State: Completed<br>Tags: <none></p>,
eventPriority: null, eventExpiration: 2016-02-25 08:35:29.912)
(February 23, 2016 8:35:29 AM)
javax.mail.internet.AddressException: Illegal whitespace in address in
string ``"<User's Name>"
This turned out to be an invalid user. Once the user was removed the initial user started receiving her build notifications again.