Unable to verify domain for SMTP

Taking a look into your question, I see that you're working through the Setup Guide but are running into an issue with completing the Email section because of domain verification. 

Now, as you may have realized, since you are using SMTP there is no need to verify the DNS records that Customer.io presents. All you need is the verification and authentication records that your 3rd-party SMTP provider requires (AWS). This is what allows their servers to send from your domain. You do, however, have to add the sending domains you want to send from to Customer.io, but you do not need to verify them.
 

• Documentation: Do I need to authenticate my domain if I’m using a custom SMTP?

That being said, I do understand that not verifying the domain prevents you from completing this section of the Setup Guide. Generally, all steps need to be completed in order to request sending approval from the Customer.io support team. Please feel free to continue working through the guide to set up campaigns and complete the other tasks. Once you start your first campaign, the setup guide will be dismissed permanently and you can then access the dashboard which will show you the campaign metrics.