No, It’s not.
When you do ‘Sender Authentication’ in the SendGrid Console, SendGrid ask you to add MX RECORD in DNS section of you domain.
This means you change your mail service.
Because, as far as I know, MX record is required only one for one domain. Otherwise, email does not know where it to be sent.
And If you change MX record without thinking, especially you’re using Gmail several years & that mail address is still officially used, your boss would call you in several hours…!
So It is much more reasonable using subdomain when you develop services without trouble.
Again, Trouble is ‘bubble doesn’t support subdomain validation of SendGrid key in the editor’. Even though It works perfectly when you use bubble API plugin. It should be fixed if bubble wants every no-coders can do SendGrid validation easily.
(SendGrid Page below)