Liviu Daia wrote,
| AFAICT, Procmail unfolds the continuation lines (presumably because the
| break points don't have any semantic relevance),
Yes, exactly.
| so the best you can do without calling an external program
| is find out if there are trailing blanks.
Trailing blanks are no indication. Consider these two examples of headers:
Fieldname: text<trailing blank>
and
Fieldname: text
<indentation>
<indentation>text
The former has trailing whitespace but no blank continuation line; the
latter has a blank continuation line but, at least by procmail's internal
egrep's opinion, no trailing whitespace.