Step 1: Step 1: Login into portal.office365.com
Sign in with your Login ID and Password.
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.