Further to http://jira.codehaus.org/browse/MPIR-189 the property does not seem to allow any markup.
This is very limiting.
HTML markup ought to be allowed, e.g. to allow a link to a page describing the e-mail etiquette etc.
In particular for ASF projects there really needs to be a link to the Forum Archive Policy http://www.apache.org/foundation/public-archives.html
The string can be specified with embedded markup if one uses the <![CDATA[ tag.
But something in the code then escapes the < & > so they appear as text in the generated page.