In a couple of recent builds, I received messages like this:

ERROR: Publisher hudson.tasks.Mailer aborted due to exception

java.lang.NullPointerException
at hudson.plugins.perforce.PerforceMailResolver.findMailAddressFor(PerforceMailResolver.java:41)
at hudson.tasks.MailAddressResolver.resolve(MailAddressResolver.java:97)
at hudson.tasks.Mailer$UserProperty.getAddress(Mailer.java:465)
at hudson.tasks.MailSender.buildCulpritList(MailSender.java:357)
at hudson.tasks.MailSender.createEmptyMail(MailSender.java:338)
at hudson.tasks.MailSender.createBackToNormalMail(MailSender.java:156)
at hudson.tasks.MailSender.getMail(MailSender.java:147)
at hudson.tasks.MailSender.execute(MailSender.java:81)
at hudson.tasks.Mailer.perform(Mailer.java:101)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:582)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:563)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:550)
at hudson.model.Build$RunnerImpl.post2(Build.java:152)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:528)
at hudson.model.Run.run(Run.java:1263)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:122)

The projects it happened for were new (in Hudson) and had rather many changes in the depot (because they're old). Some of the users involved might not exist anymore (didn't check, but it's plausible given the age of the projects).
Feel free to make the summary more accurately reflect what causes the problem (number of changes / users not existing).