Reject messages at the API & SMTP level when send limit is exceeded

It should be possible for servers to electively choose whether messages should be 'accepted and held' or 'rejected' when a server's send limit has been exceeded.

At present, all messages will be accepted.

* The SMTP server should fail to authenticate requests where the server's send limit has been exceeded.
* It should also verify the limits at the end-of-data to verify that the limit hasn't been exceeded during the pipelining of multiple messages following authentication.
* The API should return an error if the limit is reached.