Network.Mail.Mime でのメール送信方法 ref: https://qiita.com/satosystems/items/43ad35db2a289c15c998

complex.hs

{-# LANGUAGE OverloadedStrings #-}

import Network.Mail.Mime

main :: IO ()
main = renderSendMail mail'
 where
  from = Address Nothing "from@example.com"
  mail = addPart [plainPart "本文"] $ emptyMail from
  mail' = mail { mailHeaders = [ ("to", "to1@example.com, to2@example.com")
                               , ("subject", "件名")
                               ]   
               }   

simple.hs

{-# LANGUAGE OverloadedStrings #-}

import Network.Mail.Mime

main :: IO ()
main =
  renderSendMail $ simpleMail'
    (Address Nothing "to@example.com")
    (Address Nothing "from@example.com")
    "件名"
    "本文"