MTA_MAX_TO

Specify the maximum number of envelope To: addresses per message copy. If, when the message is enqueued, the number of envelope To: addresses for the message exceeds this limit, then the message will be broken into multiple copies, each copy with no more than the specified number of envelope To: addresses.

The item_length field specifies the maximum number of envelope To: addresses per message copy. By default, no limit is imposed.