I have email-ext plugin version 2.16 installed on my Jenkins. In the configure system setting GUI page, I am trying to set the Default Recipients so that all emails send by the email-ext plugin will be sent to this email. I save the settings page once done. However, when I go back in to the configure system settings page, whatever email(s) I wrote in Default Recipients has been deleted.

I know that this is a problem and that the value I set for Default Recipients did not save because all projects trying to send to the default recipients are unable to recognize that value:

Sending email to: $DEFAULT_RECIPIENTS
ERROR: Could not send email as a part of the post-build publishers.
javax.mail.SendFailedException: Invalid Addresses;
nested exception is:
com.sun.mail.smtp.SMTPAddressFailedException: 550 5.1.1 <$DEFAULT_RECIPIENTS>... User unknown

at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1196)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:584)
at javax.mail.Transport.send0(Transport.java:169)
at javax.mail.Transport.send(Transport.java:98)
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:259)
at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:243)
at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:203)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:692)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:667)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:645)
at hudson.model.Build$RunnerImpl.cleanUp(Build.java:171)
at hudson.model.Run.run(Run.java:1448)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:238)
Caused by: com.sun.mail.smtp.SMTPAddressFailedException: 550 5.1.1 <$DEFAULT_RECIPIENTS>... User unknown

at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1047)
... 15 more
Finished: FAILURE