Your primary email address in Office 365 is usually the email address you were assigned when your Office 365 work or school account was created. The primary email address could also have been changed by you or an admin at your organization. When you send email to someone else, your primary email address is what typically appears in the From field in email apps. You can also have more than one email address associated with your Office 365 for Business account. These additional addresses are called aliases.
For example, let's say Ina's primary email address is Ina@Contoso.com. She also wants to receive email at Ina.Leonte@Contoso.com and Sales@Contoso.com. You can create those aliases for her, and all the mail sent to those aliases will go to Ina's Inbox.
You can create up to 400 aliases for a user. No additional fees or licenses are required.
TIP: If you want multiple people to manage email sent to a single email address like email@example.com or firstname.lastname@example.org, create a shared mailbox. To learn more, see Create shared mailboxes in Office 365.
Add email aliases to a user
You must have admin permissions in Office 365 to do this.
1. Sign in with your Office 365 admin account at https://portal.office.com/adminportal/home
2. Select Users.
3. On the Active users page, choose the user.
4. On the user's page, next to User name / Email, choose Edit.
Note: The email alias must end with a domain from the drop-down list. To add another domain name to the list, see Adding additional domains to Office 365.
6. When you're done, choose Save. It may take 15 minutes or more for the change to propagate properly.
7. The user now has a primary address and an alias. All mail sent to Ina@contoso.com and Sales@contoso.com will go to Ina's Inbox.
Note: When the user replies, the From address will be her primary email alias.