No longer true. I was notified that some MTAs do not add the header at all so newer versions (please do NOT use anything between 0.7.10 and 0.7.13.1, there was a stupid bug!) add that header themselves. Unless you turn it off with $Mail::Sender::NO_DATE = 1

== Jenda@Krynicky.cz == http://Jenda.Krynicky.cz ==
Always code as if the guy who ends up maintaining your code
will be a violent psychopath who knows where you live.
      -- Rick Osborne, <osborne@gateway.grumman.com>