The MTA would then log a successful delivery or return a failure message to the sender as appropriate, and the filter would be responsible for affecting the delivery of the message (to the intended inbox(es) as-is, or modified to remove unwanted content, or to specific folder(s), etc.).
For very large messages, this can have an enormous impact when a decision to reject can be made as early as possible.
Moreover, unlike the former model, a milter-capable MTA can connect to multiple filters in parallel that serve specific purposes such as anti-virus, anti-spam, message authentication, flow regulation, etc.
Finally, such filters can take special action on the message: add or remove recipients in the envelope; alter the body prior to delivery; add, change or remove header fields in the message, etc.
The Sendmail Consortium provides a documented Application Programming Interface that can be used to create milters for special needs.