If you have been migrated from another platform into Microsoft 365 you will need to remove the account from Outlook and re-add it. This can be done as below:
Open the start menu and type 'control panel', once the control panel option appears click it, this will open control panel.
In control Panel select the widget for Mail
Select Show Profiles
then Add new Profile
Call the Profile M365
Enter your Name, Email address and Password, select Next
The profile should now detect your server settings and ask you to sign into a Microsoft login box 
If the Work or Home screen is displayed select work/school account
Once you have successfuly logged in this screen will be displayed 
When you have selected finish in the above image be sure top select always use this profile radio button and the M365 profile from the drop down box 
At this point select ok and close control panel. Open outlook and it will start downloading you remail. You should notice if you scroll down in the left hadn pain that any secondary mailboxes you have access to will be displayed here. You may need to enable the 'from' field from the options tab of a new message to ensure you are sending from the correct mailbox address.
For anyone not using outlook, you may have to configure your accounts manually. The server address for this is outlook.office365.com
