Sender domain is the address which recipients see in the from field when they open your emails. Eg. when you see sign up confirmation email of Pepipost, you see it coming from firstname.lastname@example.org. Here “pepipost.com” is the sender domain.
To start sending emails to your customers, you can use either your top-level domain eg. mycompany.com or you can use a sub-domain like email.mycompany.com.
To start sending emails, you need to add and verify your sending domains. The verification is done to ensure your sending domain’s security.
Go to Settings --> Sending Domains
In Add your Domains field, type the name of the domain that you want to add. e.g. mydomain.com.