We host our own email server and have discovered we have a low sending score that is causing delivery issues. From the report (senderscore.org), there is an IP address outside of the U.S. (email server is in the U.S.) that is sending mail as our domain that we have no affiliation with.
I've created an SPF record, _dmarc and set rDNS of our mail records to our actual IP. SPF and _dmark records are set to reject all not originating from our IP/MX record.
We plan on implementing DKIM as well but that is something that we have to research as it's did not seem as straight forward as the SPF and DMARK records.
Is there anything else I can do short term to target this IP Address from sending email as our domain and hurting us?