I was wondering why a lot of Chinese spam was ending up in my inbox.

$ grep qq.com -lr . | uniq | xargs rspamc learn_spam

Turns out, because these spam mails don't really have bodies, most of them weren't learned as spam in rspamd's bayes classifier, and they weren't considered spammy enough:

HTTP error: 400, <...@qq.com> contains less tokens than required for bayes classifier: 2 < 11