Hi
Sorry for the latest reply. The issue is still a problem.
In order to reproduce it, you need to configure an empty job with extended email configuration and to enter email address in the success recipient list (but leave the global recipient list empty).
Here is the exception that appears in the console output of the job:
Email was triggered for: Success
There are 1 triggered emails.
Sending email for trigger: Success
ERROR: Could not send email as a part of the post-build publishers.
javax.mail.internet.AddressException: Illegal address in string ``''
at javax.mail.internet.InternetAddress.<init>(InternetAddress.java:94)
at hudson.plugins.emailext.ExtendedEmailPublisher.createMail(ExtendedEmailPublisher.java:287)
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:249)
at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:241)
at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:199)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:541)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:527)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:515)
at hudson.model.Build$RunnerImpl.cleanUp(Build.java:158)
at hudson.model.Run.run(Run.java:1218)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:123)
P.S. I forget to mention that no email has been sent 
This was fixed by againjj in r15195.
There are many unreleased changes in email-ext, including this fix.. I'll look into putting out a new release.