Hi Nicolas. I fixed the "Mail object returned error:" from ContactPage on my site. Don't know if your error was for the same reason, and I'd still like to find a better fix, but at least I don't get the error anymore :). See: http://www.mediawiki.org/wiki/Extension_talk:ContactPage#mailing_error
This post was posted by Nickenge~mediawikiwiki, but signed as Nickenge.