I have found that POPFile is quite good at differentiating good and bad email from the same source.

I get a regular weekly newsletter from a computer magazine which POPFile always marks as "good". However one week I was surprised to find a message from that magazine marked as "spam". When I looked at the message I found it was offering discounted subscriptions to some other (non-computer) magazines from the publisher of the computer magazine. Although this message was similar to the regular newsletter (some text, some graphics, various links to websites) the content was different enough for POPFile to decide it was not one of the regular newsletters.

You have said that the good email is "seldom" and the rest is "often". This means it may take a while for POPFile to learn how to differentiate between these two types of mail from the same source. It is possible that feeding some previously received "good" email from that acquaintance into POPFile would reduce this learning period.

However it may be easier and quicker to use the filters provided by your email client:

(1) if ("From" my acquaintance) and ("To" contains my address) then move to Inbox
(2) if ("From" my acquaintance) and ("Cc" contains my address) then move to other folder

Brian