How to setup DKIM for

I have setup website masterfish. net with mail domain. SPF record, DKIM and DMARC record been setup and pass all tested. Reports from google, yahoo or outlook are showing 100% pass. I meet problem when system/root (Ubuntu 22.04LTS) start using mail server to send information to me as does not use masterfish. net domain but name of the server mail.masterfish. net. This cause sending new reports from email providers with fail tests for SPF, DKIM and DMARC. I have create SPF and DMARC on DNS but DKIM for masterfish. net does not work with mail.masterfish. net.
Is it any way to create DKIM for mail.masterfish. net or make system to send email as instead