An email address identifies the account that messages should be delivered (e.g., firstname.lastname@example.org). An account can have multiple email addresses. Each account in Microsoft 365 has only one primary address, but an account can have multiple alternate addresses. An account will accept email for any email address on the account.
In email addresses, the domain is the part of an email address comes after the @ symbol. For example, for the email address of "email@example.com", the domain is "wisc.edu".