Spam sent to domain by domain

Hi guys,

Having an issue where we are getting constant spam on several domains within the same domain with a non-existent email. For example, info@domain.com exists, but we are receiving emails to info@domain.com by tickets@domain.com or a random email (ie: randomname@domain.com)

The spam is non-stop. Also, checked the header and it matches with @domain.com. In the header will show in message id:...

Spam sent to domain by domain