Email routing not working - "relay access denied"

Everything appears to be configured properly and enabled in Cloudflare, yet when I test the email address I have routed, I get the following error msg every time:

Your message couldn’t be delivered to [email protected] because the remote server is misconfigured. See technical details below for more information.

“The response from the remote server was: 554 5.7.1 [email protected]: Relay access denied”

I have added and double-checked the MX, TXT & dmarc records provided by Cloudflare. My website is up and running with no issues on the domain.

Routing Status: Enabled
DNS records: Email DNS records configured

I have verified and confirmed the destination email address.

I have catch-all disabled.

I am at a loss.

Any ideas?