I've seen a bunch of messages where Nabble is sending HTML tags as part of the quoted section of responses through to plain-text mailing lists. For instance:
http://n2.nabble.com/Installer---work-in-progress-td2493651.html#a2524186vs.
http://lists.osgeo.org/pipermail/mapguide-internals/2009-March/003419.htmlHelp?
Jason