Need to restrict Outgoing emails being sent by Attackers from my Domain

I have a domain with two mail servers. 1. Google mail (G Suite) server which contains some mailboxes and also acts as a relay to another cpanel mail server. I got many complaints that emails are being sent by my domain by some unkown attackers. I need to fix this issue asap. I reckon that this has to do with DKIM and DMARC not being set up. You need to effectively stop these emails from being sent at all with whatever action that needs to be taken. Emails received are uploaded in the attahment.