Comment-folded whitespace. It's the little things in parentheses (e.g., the timezone name is usually included in a comment in the Date: header). It's also the bane of anyone who has ever implemented an email message parser.

That makes sense, thank you.

This StackOverflow replay happens to answer this question exactly, the actual definition of "CFWS" is near the bottom of the text:

https://stackoverflow.com/a/42193413/544779

It's just an excerpt of the spec, but I link to it instead of to the spec because I think it's easier to read (formatting/colors) and it has links to the spec anyway.

Ahh, thank you, I missed that part of the spec. The "folding" part was what confused me, thanks again.