Using SIEVE to Discard Mail

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When I use SIEVE to discard or jettison email, is the email physically no longer on the server?

Yes, this is the default behavior. Discarded and jettisoned messages are sent to the bitbucket channel by default and the bitbucket channel in turn sends all messages to /dev/null unconditionally.

However, there are two MTA options, FILTER_DISCARD and FILTER_JETTISON, that respectively control where discard and jettison send messages. The default value for both options is 1, which means discarded or jettisoned messages will be sent to the bitbucket channel. Either option can also be set to 2, which means that discarded or jettisoned messages will be sent to the filter_discard channel instead. Messages wait in the filter_discard channel until they are either manually moved to another channel queue for delivery or else they reach the notices limit on the channel. No warnings are ever sent for messages queued to filter_discard and no nondelivery notifications are ever returned.