550. 5.7.1 Unconfigured Sending Domain

A sending domain is a domain that is used to indicate who an email is from via the "From:" header. Using a custom sending domain enables you to control what recipients see as the From value in their email clients. DNS records can be configured for a sending domain, which allows recipient mail servers to authenticate your messages. The Sending Domains API provides the means to create, list, retrieve, update, and verify a custom sending domain.
 
Improperly Configured Sending Domain

If you receive the following error message "550 5.7.1 Unconfigured Sending Domain" as a response when sending a transmission, it is because you used a from address whose domain is not one of the sending domains that you created in your account.
 
Example
If I use "sales@example.com" as my message's from address, but the only sending domain in my account is autos.example.com, I will receive a 550 5.7.1 error message when I make a transmission with the REST API or SMTP relay.
 
Solution
Changing the from address to "sales@autos.example.com" will correct the error, because autos.example.com is a configured sending domain.
 
Additional Information

Refer to this Knowledge Base Article to learn more about how to configure Sending Domains.