max_size rule too pessimistic - evaluates the message size before filtering

As noted in #368 (closed) by @msapiro, the maximum size rule for posts is evaluated based on the size of the message as received.

This is too pessimistic, as it does not account for filters that strip attachments, multipart/alternative, etc.

Presumably it also doesn't account for anything added to a message - I assume a processor could add an image or even text that would push a message over the max_size threshold...

In any case, the maximum size rule should be applied to the message after all processing.

The remark in #368 (closed) has this diagnosis:

There is another issue regarding max-size in that the test is based on the incoming message size before any content filtering (the specific message here was too big because of an HTML alternative part that was ultimately collapsed out). This can be addressed in MM 2.1 by reordering the pipeline, but the separation of rules and handlers in MM 3 doesn't allow that approach.