Hi @TeslaGX, 

Welcome to the Community! Happy to explain this further! 

When setting up a dedicated sending domain, you will need to pick a unique domain, in essence, one that hasn’t been used already. In fact, DMARC is a protocol designed to give domain owners the ability to protect their domain from unauthorized users sending emails from their domain, that’s probably why this was rejected. 

Do you mind sharing why you’re trying to use a Shopify sending domain from your brand? I would recommend using your brand’s name so at to more easily set up. Additionally, I’d suggest checking out our documentation on how to choose a subdomain for dedicated sending as well as it might also be helpful to your situation. 

Finally, I’d recommend checking out a similar thread that might be helpful when setting this up for your account to gain more insight.

Thanks for your participation in the Community!

-Taylor