Although an account in Office 365 can have multiple email addresses, each account has only one "primary address". This document explains what a primary address is, how it works, and what factors you should consider when selecting your primary address.
Please refer to the following document if you need to Change your Primary Address
How is my primary address different from the other email addresses that are associated with my account?
If there are multiple email addresses associated with your account (e.g., firstname.lastname@example.org, email@example.com, and firstname.lastname@example.org), you will need to select one email address as your primary address. Although you can change your primary address later, you can only have one primary address assigned to your account. The differences between a primary address and an alternate address are outlined below:
All email addresses associated with your account:
If an email is sent to any email address associated with your account, that email will be delivered to your Office 365 account.
If a calendar invitation is sent to any email address associated with your account, that calendar invitation will be delivered to your Office 365 account.
Only your primary address:
All sent mail will appear to come from your primary address.
All calendar invitations you send will appear to come from your primary email address.
Only your primary email address will be listed in the directory search feature within Office 365 (the Global Address List or GAL).
Alternate address(es) will not be listed in the directory search feature in Office 365 (the Global Address List or GAL) - only your primary address will be listed. Change primary address.
Please keep the features above in mind when selecting a primary address for your account.