The current configuration doesn't work when moving spam from the INBOX
to Junk on a local maildir and then syncing the result to the IMAP
server with `mbsync(1)`. This is because `mbsync(1)` doesn't support a
mvoe-detection[1] (i.e. an IMAP MOVE which subsequently causes a Sieve
COPY according to RFC6851 which then triggers report{h,sp}am.sieve), but
instead sends `APPEND` (and removes the message in the src mailbox after
that).
Tested on my own mailserver that this fixes spam learning.
This doesn't work the other way round though because `APPEND` doesn't
have an origin. However, learning mails as spam happens more often than
learning spam as ham, so this is IMHO still useful.
[1] https://sourceforge.net/p/isync/mailman/isync-devel/thread/87y2p1tihz.fsf%40ericabrahamsen.net/#msg37030483
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