I expect it means that the header information can’t narrow the email origin down to one domain name, and the spam filter is programmed not just block multiple domain names when it finds more than one in the header, to reduce users blocking more domains than they really intend or understand they are doing.

Does it let you block domains without referring to an email? If so, you could look at the header details yourself, and add filters for the ones that you’re sure you want to never get email from.

Thanks. I will have to check it out.

Sure. Oh, another major reason that would make sense for them to program it that way, is a great many spam and pfishing emails deliberately use an innocent domain name (like that of a company they think might send you legitimate email), as well as whatever they’re saying they’re using as a mail server.

So if they blocked all domains in the headers, you’d be blocking your bank, or microsoft, or whoever they’re pretending to be or use for a mail host.