You must ensure that secure connections are selected for both incoming mail and for outgoing mail.
Multiple email accounts can use the same outgoing SMTP server, therefore SMTP servers are created separately from email accounts.
In the Account Settings dialog left pane, select Outgoing Server (SMTP) from the bottom of the list. There may be an entry in the resulting list that was created when you entered your account details, either edit that or add a new entry with the following settings:
- Description: Nexus365
- Server Name: outlook.office365.com
- Port: 587
- Use name and password: Checked
- Username: Your SSO plus @OX.AC.UK (for example abcd1234@OX.AC.UK)
- Use secure authentication: Checked
- Connection security: STARTTLS
Mozilla also provide detailed IMAP configuration instructions for Thunderbird