My thought would be a routing issue of some kind, but I am unable to verify anything.
When I tried to look at the domain, I found that there are no DNS records for it. There is a whois record, but event that is very sparely populated.
"pvsyst.com" or "www.pvsyst.com"?
pvsyst.com : http://toolbar.netcraft.com/site_report?url=pvsyst.com - Few details
www.pvsyst.com : http://toolbar.netcraft.com/site_report?url=www.pvsyst.com - OK
Hayton is close
ping: cannot resolve pvsyst.com: Unknown host
PING www.pvsyst.com (220.127.116.11): 56 data bytes
A host lookup show the truth:
pvsyst.com mail is handled by 10 mta-gw.infomaniak.ch.
Conclusion: there is no host such as pvsyst.com.
Browser will try to add www. as prefix to an unresolvable host and that's why it is mostly working in the browser w/o MWG. WMG tries to resolve and will then deny the request, you should get a proxy error in this case. As the browser receives a response (from the proxy) it will ot try adding a www. as prefix.