A 550 5.7.1 rejection means the receiving server refused the message for policy reasons. In cold email the cause is nearly always authentication.
Work through the DNS checklist
Confirm the SPF record exists and includes the sending infrastructure, that the DKIM key is published and verifies for the sending domain, and that DMARC is published.
Re-run verification after changes
DNS changes propagate within hours. Re-run the domain verification in MunchReach afterwards so the mailbox can return to an active state.
Was this article helpful?