According to https://issues.jenkins-ci.org/browse/JENKINS-11665, global recipients variable does no longer exist.
But is still use in email-ext plugin, even if there is not present. $PROJECT_DEFAULT_RECIPIENTS is added to recipient list automatically.

Stacktrace:
Email was triggered for: Success
Sending email for trigger: Success
Sending email to: www.test@xxxxxxx.com $PROJECT_DEFAULT_RECIPIENTS
ERROR: Could not send email as a part of the post-build publishers.
com.sun.mail.smtp.SMTPSendFailedException: 250 2.0.0 pA99kJg2007424 Message accepted for delivery;
nested exception is:
com.sun.mail.smtp.SMTPAddressFailedException: 550 5.1.1 <$PROJECT_DEFAULT_RECIPIENTS>... user unknown
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:598)
at javax.mail.Transport.send0(Transport.java:169)
at javax.mail.Transport.send(Transport.java:99)
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:260)
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:695)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:670)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:648)
at hudson.model.Build$RunnerImpl.cleanUp(Build.java:172)
at hudson.model.Run.run(Run.java:1448)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:47)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:230)