Have a look at the message "source"; select the message and type ctrl+u.

Many of these messages are set up to include a lot of random text, usually hidden from view. I have see samples from biographies, recipes, travelogues and so on.

Thunderbird's Junk Controls work by counting the appearances of words, and looking for correlation between high word frequencies and the messages you train it to see as good and bad. So if a message has a high proportion of words associated with spam then the message will be scored as spam-like. Large tracts of innocent text inserted into a message work by diluting the spam-like ingredients with a lot of un-spammy words.

I have seen html comment tags used to convey the camouflage text and you might think it would be quite easy to identify messages that had a large amount of comment notation. But this is not the only method used.

I don't know how you filter these messages. It's trivial for a human user to glance at the source and spot non-relevant text. That's a hard concept to turn into an algorithm. :-(

Note that the Junk Controls don't pay any attention to the "sender". The From: address in true spam is always spoofed and usually stolen. It would be a very foolish spammer who sent messages from his own computer using his own email address.

If you are getting repeat messages from one particular sender then first of all I'd ask if this is truly "spam"? One definition of spam is "unsolicited commercial email", abbreviated to UCE, and many people seem to think that unwanted messages are spam, even if at some point in time they signed up or registered at a website and accepted that messages would be sent to them. In this type of case, you should look for an option to unsubscribe.

If it really is coming consistently from a particular sender, then I would create a filter to deal with it explicitly. This is not a situation that the Junk Controls filter was designed to cater for.