If Gnus tried to match some of the regexps used in splitting against
   extremely long header lines, the match attempt would take far too long
   to be tolerable.
   
   To deal with this, Gnus excludes long header lines from the function
   that does the splitting. If a mail message has, for example, a To:
   header longer than 1024 characters, that line will not be considered
   for splitting purposes.
   
   Very recent versions of Gnus (pgnus, thus all official versions >=
   5.8) deal with this a bit better. In those versions, the long lines
   are truncated instead of excluded, and there is a variable,
   nnmail-split-header-length-limit, to control the threshold for
   truncation.