Zimbra learns ham and spam by sending it to certain mailboxes. For our setup, this doesn’t work (easily), because our server is configured to always send mail to another SMTP server and not do any local delivery. I did that, because our zimbra server is not actually on the domain it thinks.

To disable the learning accounts, I did this: 

zmprov mcf zimbraSpamIsSpamAccount ''
zmprov mcf zimbraSpamIsNotSpamAccount ''
zmcontrol stop
zmcontrol start

I didn’t delete the accounts, so I can enable it later.

To enable it, I guess I have to configure these two accounts on our hosting provider’s servers, fetch and deliver them to Zimbra and it works. I’ll do that some time…