Private Message across multiple sites

[Mythotical]

Ok here is the deal, I have seen it here somewhere but for the life of me I can't find it, I searched but my eyes are hurting from looking at soooo many threads.

Anyway, just to be specific, the mod would allow a user to PM say 'userA' from 'siteA', when 'userA' logs on to 'siteB' they will see they have a new PM so they access it from 'siteB', well when they visit 'siteA', it shows as read instead of unread.

Any help much appreciated.

Thanks in advance
Steve