What does it mean to have the domain set as the "Default" sender? It is good practice to have a default verified sender domain. Our system will use the default verified sender domain if a non-verified domain is used for the email for the account. This is particularly important if sub accounts are using the main account's sender domain or for resellers that want notification emails to send using their domain.
You can set your domain as the default sender on your Settings>Domains screen:
You will know the domain has been set as the default successfully when the star beside it's name turns blue.
You can find out more about verifying domain in our other article How to verify your domain