Also had to revert to 2.15 while waiting for this regression to be fixed

I'm running Jenkins 1.448 and Jenkins Email Extension Plugin 2.16 and see this behavior as well. I did not see it happening with Jenkins 1.437 and Jenkins Email Extension Plugin 2.15.

I did find that if I remove the trigger and then add it back PROJECT_DEFAULT_RECIPIENTS is removed from my config.xml. So Perhaps all one needs to do is edit all config.xml files. It would probably be better if email-ext upgraded the settings itself though.

Just editing config.xml and removing PROJECT_DEFAULT_RECIPIENTS didn't fix it. I restarted Jenkins and that left me with an empty recipient list message all of the time. I removed all of my triggers, saved the config, then added the triggers back and now it sends email properly. Hope this helps debug the issue.

or try this
find ./jobs -type f -name "*config.xml" | xargs grep -l "PROJECT_DEFAULT_RECIPIENTS" 2>/dev/null | xargs sed -i 's/PROJECT_DEFAULT_RECIPIENTS/DEFAULT_RECIPIENTS/'

No that doesn't work. That results in the error below. Something else needs to be modified in the setup.
16:03:07 Recording test results
16:03:08 Email was triggered for: Success
16:03:08 Sending email for trigger: Success
16:03:08 Sending email to: $DEFAULT_RECIPIENTS
16:03:09 ERROR: Could not send email as a part of the post-build publishers.
16:03:09 javax.mail.SendFailedException: Invalid Addresses;
16:03:09 nested exception is:
16:03:09 com.sun.mail.smtp.SMTPAddressFailedException: 501 <$DEFAULT_RECIPIENTS>: recipient address must contain a domain
16:03:09
16:03:09 at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1196)
16:03:09 at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:584)
16:03:09 at javax.mail.Transport.send0(Transport.java:169)
16:03:09 at javax.mail.Transport.send(Transport.java:98)
16:03:09 at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:263)
16:03:09 at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:247)
16:03:09 at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:207)
16:03:09 at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
16:03:09 at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:700)
16:03:09 at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:675)
16:03:09 at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:653)
16:03:09 at hudson.model.Build$RunnerImpl.cleanUp(Build.java:171)
16:03:09 at hudson.model.Run.run(Run.java:1453)
16:03:09 at hudson.matrix.MatrixRun.run(MatrixRun.java:146)
16:03:09 at hudson.model.ResourceController.execute(ResourceController.java:88)
16:03:09 at hudson.model.Executor.run(Executor.java:238)
16:03:09 Caused by: com.sun.mail.smtp.SMTPAddressFailedException: 501 <$DEFAULT_RECIPIENTS>: recipient address must contain a domain
16:03:09
16:03:09 at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1047)
16:03:09 ... 15 more

Working on a fix for this.

According to the value displayed in the Jenkins configuration it should be possible to use $DEFAULT_RECIPIENT_LIST instead

Code changed in jenkins
User: slide
Path:
src/main/java/hudson/plugins/emailext/EmailExtensionPlugin.java
src/main/java/hudson/plugins/emailext/EmailRecepientUtils.java
src/main/java/hudson/plugins/emailext/EmailType.java
src/main/java/hudson/plugins/emailext/ExtendedEmailPublisher.java
src/main/java/hudson/plugins/emailext/ExtendedEmailPublisherDescriptor.java
src/main/java/hudson/plugins/emailext/plugins/ContentBuilder.java
src/main/resources/hudson/plugins/emailext/ExtendedEmailPublisher/config.jelly
src/test/java/hudson/plugins/emailext/ExtendedEmailPublisherTest.java
src/test/java/hudson/plugins/emailext/plugins/ContentBuilderTest.java
http://jenkins-ci.org/commit/email-ext-plugin/09296a6d5e321ea6a553b0ff1de58a6b165d13c7
Log:
Merge pull request #27 from slide/master

Fix JENKINS-11683, JENKINS-1144, JENKINS-11665

Compare: https://github.com/jenkinsci/email-ext-plugin/compare/3789acf...09296a6

Fixed in 09296a6