We are seeing JENKINS-20133 which is fixed in 2.36.
As mentioned in #301 there will be a 2.37 soon so we should wait for this.
I will work on that, so we can get the new not-blocking feature when sending out emails.
Upgrade EmailExt plugin to v2.38.1 (#345)
Meanwhile v2.38.1 is available. So I will upgrade to this version.