Suppose we have a website www.abc.com. We want to send (one way) system emails to users (for email verification, password reset etc) which may be sent from firstname.lastname@example.org and email@example.com. We also want our employees to have email addresses (two way) like theirName@abc.com (HR guys, customer-support guys etc), using which they can send and receive emails.
I am planning to look into sendGrid (system mails) and google (employee mails).
What I don't understand is that how the mappings are done? Like, when a mail is sent by some customer to employeeName@abc.com, how will it be sent to an employee's mail rather that being listened by sendGrid? And while sending mails from our side, how sendGrid is able to map to our domain? I want to know where and how all these mappings exist.