KAZOO-3157

https://github.com/2600hz/kazoo/blob/master/deps/gen_smtp-8f7ba01172e679e8b4262334cefc3451b4c36777/src/mimemail.erl#L44-47 seems to indicate it should at least be a binary though and https://github.com/2600hz/kazoo/blob/master/deps/gen_smtp-8f7ba01172e679e8b4262334cefc3451b4c36777/src/gen_smtp_client.erl#L260 seems to indicate that gen_smtp_client also expects a binary.