I'm surprised you willingly burn your own domain's reputation. Other email-as-a-service companies require a validated domain with SPF and DKIM in the DNS TXT re

I'm surprised you willingly burn your own domain's reputation. Other email-as-a-service companies require a validated domain with SPF and DKIM in the DNS TXT records or use a burnable subdomain, e.g. freeaccount12345.sendgrid.com; or even better, 12345.sendgridfreeaccount.com.