When email.policy.SMTP.fold() attempts to wrap a line in which a chunk that does not contain a space is longer than the maximum line length, two things go wrong:
 - The second line does not begin with a space, creating an invalid header.
 - The space before the long chunk is stripped, causing the line break to occur in an invalid place.

In the attached test case, email.policy.SMTP.fold() is called on a line of the style:
Content-Disposition: attachment; filename="<filename>"

<filename> does not contain any spaces and increases in length with each pass. Everything works correctly when 'filename="<filename>"' fits onto one line. However, once it is longer than the maximum line length, the following happens:
 - the space after the semicolon is dropped
 - the line break often splits up the 'filename' keyword
 - the second line loses the initial whitespace