If I want to send to 4 places for each message, then I can only send 4 different messages.
Right. The addresses are hard-associated with the messages in each of the 16 message slots.
What I do to overcome this somewhat, for non-critical alarm situations --
(1) Send the "real" message to my "regular" email address which has the full content; there are several of these, one for each alarm condition
(2) Send a "referrer" message to my other email account(s) saying in essence "refer to the real message for details"; there need be only one of these defined for each account (this is where the savings occurs)
Burglar and fire messages are sent immediately and in full to all relevant accounts.
This won't be satisfactory for every situation, but it is a way to get some relief from the constraint.