You should be worried if you see a high number of emails sent from the IP addresses unfamiliar to you but using your domain. Make sure SPF, DKIM and DMARC pass for the messages sent by your legitimate sources. When everything is good with your sending sources, set the p=reject policy in your DMARC record to block any message sent from your domain which fails the DMARC validation.
Updated on October 1, 2020
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