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.