cPanel’s Exim mail server is not a full outbound relay resolver; if a domain exists locally, Exim will try and deliver locally (or reject if no mailbox exists). It does not consult external MX records when it believes the domain is hosted locally.

Both domains exist in cPanel (so Exm sees them as local).
Cloudflare handles incoming mail routing (MX record)

Outbound email is being sent through the same instance, resulting in "550 No Such User" because Exim tried local delivery before consulting the MX for the recipient domain.

To fix this, the email server must treat those domains as remote for delivery, even though they are hosted on the same server.