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Now using struct smtp_address rather than sieve_address.
In some cases, struct message_address may prove to be more suitable, but this is the best fit for now.
Adjusted to the renamed fields in struct mail_deliver_context.

Added support for getting message part information and content.

lib-sieve: mime extension: Fixed the header :mime :anychild test to work properly outside a foreverypart loop.

lib-sieve: Created special stringlist class for message headers, so that header names can also be returned by the iterator.

This makes this available for other extensions.

This allows extension of test commands that evaluate the message somehow.
Currently this is implemented only for the message header.
Extensions can thereby influence what headers are evaluated and how.

  - Built generic interface for alternative script sources.
  - Implemented dict script location.
  NOTE: ManageSieve will not work with this yet, nor will sieve_before/sieve_after.

This is needed for the new extprograms plugin.

Incorporated distinction between original and final envelope recipient in Sieve interpreter, as recently introduced in Dovecot.

Major rework of envelope address handling: a normalized version of the envelope addresses is maintained in the message context and message context is now also available during action execution.

Body: added test regarding empty bodies and fixed testsuite to flush the message context when the message is changed.