Publish the mail provider's MX records with explicit, distinct priorities, and keep a forwarding host only as a temporary migration control. The deciding constraint for a fintech product is evidence: when a customer says mail disappeared, the on-call engineer needs to reconstruct where an SMTP sender was told to go, which system accepted the message, and whether the domain separately authorized outbound mail. Short answer: an MX cutover is ready when six checks agree: the intended records exi...