Discard vs. DSN

If 'discard' MUST NOT cause failure DSN generation, this could be difficult
in certain plug-in environments (such as one of the platforms where my
Sieve implementation runs).

We can either (1) make 'discard' optional, (2) say that 'discard' SHOULD
NOT cause failure DSN generation, or (3) say this is just Too Bad.

Personally, I suggest (2).