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

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?