I have read the documentation but cannot understand how to customize the email body. I am an experienced Perl software developer, but I have no experience of jelly or groovy. I understand the principle but the instructions are lacking in detail.

What I've tried:
In the "Pre-send Script" field I have put: ${SCRIPT,template="hello.groovy"}
In the file /var/jenkins_home/email-templates/hello.groovy I have put:
<%
println "Hello world!"
%>

When I test this in the "Email Template Testing" page, I get the result: Hello world!

But when I try it in a real job, I get the error below in the console:

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
Script1.groovy: 1: unexpected token: world @ line 1, column 7.
Hello world!
^
1 error
(stack trace snipped)

Could a minimal "hello world" script be provided which demonstrates how to define an email body?