For me, rspamd with corresponding exim-acls practically solved that issue since 02/2016. Spam is mostly rejected, otherwise tagged (and filtered with dovecot-sieve downstream into a review-box) or - if borderline - greylisted. Have had no false positives I'm aware of and only 1-2 false negatives per week slip through that cascade.
edit: Although some training (i.e. manually moving spam to IMAP-Junk) was necessary during the first few months.
edit: Although some training (i.e. manually moving spam to IMAP-Junk) was necessary during the first few months.