Step 1: Step 1: Login into portal.office365.com

Sign in with your Login ID and Password.

Continue…

Step 2: When entering account click on Admin as shown into the picture.

Step 3: In the Office 365 admin center, choose the Active users option.

Step 4: Click on the Username and click on the Mail.

Step 5: Now click on the Manage Mailbox Permissions.

Step 6: Click on the Edit button against the Send.

Step 7: Click on Add Permission and click on User and click on the Save button.

Step 8: After click on Save our user is saved now.