class Mail::Message

def resent_from=( val )

mail.resent_from #=> ['mikel@test.lindsaar.net', 'ada@test.lindsaar.net']
mail.resent_from = 'Mikel , ada@test.lindsaar.net'
mail.resent_from #=> ['mikel@test.lindsaar.net']
mail.resent_from = 'Mikel '

Example:

Sets the Resent-From value of the mail object, pass in a string of the field
def resent_from=( val )
  header[:resent_from] = val
end