On a failed build, the following message is shown in the build log when the build comes to sending email:

Error substituting with token-macro plugin: org.jenkinsci.plugins.tokenmacro.MacroEvaluationException: Unrecognized macro 'ConnectionHandler' in '[here comes html mail filled template]'

The email itself is sent to the recipients. The issue also does not occur on successful builds.

It also seems that the issue is not always present for the failed builds. Sometimes mail goes without any errors. It also seems that this 'unrecognized macro' also varies; I just found another occurrence complaining about unrecognized 'AbstractBuildExecution'

I am using globally configured email-ext with content: ${JELLY_SCRIPT, template="html"}