We struggle on this issue too and in my opinion this ist not a minor issue, it's more an incident! Actually the issue is some kind of a show stopper as the user did not receive proper notifications anymore.

We also struggle on this issue, any progress ?

Code changed in jenkins
User: George
Path:
src/main/java/hudson/plugins/emailext/ExtendedEmailPublisherDescriptor.java
src/main/java/hudson/plugins/emailext/plugins/recipients/RecipientProviderUtilities.java
src/main/resources/hudson/plugins/emailext/ExtendedEmailPublisher/global.groovy
src/main/webapp/help/globalConfig/allowUnregistered.html
http://jenkins-ci.org/commit/email-ext-plugin/19081b2aa3d32c479125b538c545fdd3065960f8
Log:
JENKINS-43268 adding global checkbox to allow sending to unregistered emails. This is a workaround to a SCM plugin issue, like JENKINS-9016 and probably others (#161)

Would it be possible to have some kind of 'domain filter' when you enable the 'Allow sending to unregistered users' checkbox?

For instance: allow sending mails to contributers to a git repo that don't have a Jenkins account, but that do have an email address set up that ends with '@domain.com'.

This would be interesting so that when we're using open source git(hub) repositories we don't accidentally sent out mails to the contributers there, when one of our builds fail.

Can't the e-mail address(es) in the commit message be used to find Jenkins security realm users?

The e-mail address in my commit messages is the same as the e-mail address associated with my Jenkins account.  Why can those not be matched up?