This information will allow you to set up the incoming and outgoing mail servers of your mailbox. To set up your Microsoft 365 mailbox on your laptop, tablet or mobile device, or to be able to use your webmail, you will need the settings of your Microsoft 365 mail account. Retrieving your Microsoft 365 mailbox settings from Outlook online
If, like me, you hate the in-built mail app, try Thunderbird instead.
Check and see if the issue is reproducible. Your Microsoft 365 (Office 365) package includes an Exchange e-mail account. Rule in/out Outlook/Office Configure the users mailbox using the in-built Mail app in Windows 10.