What causes mails to get striked-over only, and not deleted?

[Sebastian Nielsen]

I have tried with this sieve config:

  imapsieve_mailbox1_name = Sent
  imapsieve_mailbox1_causes = COPY APPEND
  imapsieve_mailbox1_after = file:/etc/dovecot/sieve/sent.sieve

and this sieve file (sent.sieve):

discard;


This should clearly cause the mail to be deleted right?
But whats happen, is that the mail is not deleted, its just marked for
deletion (gets a strike-through in Microsoft Outlook).

(have tried with _before too, but it wont help).

What can I do to ensure the mail actually gets deleted (completely, not
moved to trash) and not just flagged for deletion?

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