The groovy-html template does not correctly encode getLog() content as HTML in the email. For example, less than (<) and greater than (>) characters, which are used frequently in junit failure messages sometimes disappear in the email messages.

If you look at https://github.com/jenkinsci/email-ext-plugin/blob/master/src/main/resources/hudson/plugins/emailext/templates/groovy-html.template

You'll see

<% build.getLog(100).each() { line -> %>
<TR><TD class="console">${line}</TD></TR>
<% } %>

The $line needs to be HTML encoded.

EDIT: I checked again and maybe this isn't an issue but an issue with a mac Mail client. Alex, can you check and mark as no repro if necessary? Thanks.