there is no support for send_at in MandrillTemplatedMessageRequest

[nati159]

see http://help.mandrill.com/entries/24331201-Can-I-schedule-a-message-to-send-at-a-specific-time-

https://mandrillapp.com/api/docs/messages.JSON.html#method=send

mandrill has an API for scheduling, canceling and rescheduling message.