class Mail::Message

def smtp_envelope_to=( val )

mail.smtp_envelope_to #=> ['mikel@test.lindsaar.net', 'lindsaar@test.lindsaar.net']
mail.smtp_envelope_to = ['Mikel ', 'Lindsaar ']

mail.smtp_envelope_to #=> 'mikel@test.lindsaar.net'
mail.smtp_envelope_to = 'Mikel '

Example:

Sets the To addresses on the SMTP Envelope. 
def smtp_envelope_to=( val )
  @smtp_envelope_to =
    case val
    when Array, NilClass
      val
    else
      [val]
    end
end