Improve Deliverability With a Custom Sending Domain

To improve deliverability, you can add and verify your domain by using your email address as the sender. You can manage your domains under the Account > Sending Domains section. This process involves adding DNS records in the domain service that hosts your domain.

A common issue is incorrectly adding the subdomain. When you add the required subdomains, you usually only enter the subdomain part, not the entire domain. Instead of entering em1235.yourdomain.com, you would usually only enter em12345 because  yourdomain.com is implied since it's the domain you're adding it to. 

If you enter the full domain, then it will register  em1235.yourdomain.com.yourdomain.com, notice the duplicate domain name. How this is added is different depending on which DNS provider you use.