The RFC822 standard specifies how long header lines are to be folded for
better readability, to conform to a maximum line length, etc.  It provides
for breaking fields at well-defined syntactic points and a header line
continuation convention, which is that a header line beginning with blank
or horizontal tab is considered a continuation of the preceding line.
Mailers already "unfold" header lines according to this standard.  This
would be the appropriate way to handle long header lines without having to
invent a new convention using backspace.