Don’t worry, I’ll try to guide you as best as possible, I’m not an expert on the subject, but so far my email works without problems, personally I use “sendiblue”, because it was the only one that allowed me to use an account without going through filters that others put.
In short, the section that I mention is in “networking”.
1 Clic on Networking
2 Enter your domain “example.com” NOT "talk.example.com
3 Choose the project
Personally, it was necessary for me to add and manage the domain name from “DO”, since Sendinblue asked me to add some dns values to verify domain ownership, just like google for user registration.