Welcome to WebmasterWorld Guest from 188.8.131.52
Double-checking here, because it sounds like a configuration I met recently on an unrelated site:
Your primary domain is example.com
Your vendor is at spam.example.com Well, maybe not literally, but work with me here
The primary domain is hosted on your own server, or at least in your own space within shared hosting
The subdomain is physically located in a completely different place, so requests for the subdomain will never see the primary domain's htaccess and/or config.
Is that the setup?