We have run into an issue where jobs on mac nodes are occasionally hanging on the email trigger.
This doesn't happen on all of our OSX based jobs, but occasionally on some of them. There is not relation to which jobs it happens it, on which hosts, nor with how many people are being notified on the email. On jobs with dozens of people it can happen fine, but on jobs with one or no people it can hang anywhere from 5-15 minutes.
It looks like it might be related to
JENKINS-14755 but that issue was fixed years ago.