rfc2822 April 2001

4.2. Obsolete folding white space

   In the obsolete syntax, any amount of folding white space MAY be
   inserted where the obs-FWS rule is allowed.  This creates the
   possibility of having two consecutive "folds" in a line, and
   therefore the possibility that a line which makes up a folded header
   field could be composed entirely of white space.

   obs-FWS         =       1*WSP *(CRLF 1*WSP)