Unknown Chinese mail accounts

[hillsea]

I am using an external SMTP authentication and I just found out that some unknown Chinese mail accounts is using my Zimbra Mail server to send messages. Said accounts are nowhere found in displaying accounts list in zmprov, even hidden ones. Can anyone redirect me to some bug or solution I might be missing? My version is 8.8.15

[phoenix]

The first thing I would do is to check with an online service to see if your server is an open relay.