Organisation-Wide Email Security Upgrade – Transition to Microsoft Defender for Office 365
We are pleased to announce an important enhancement to our email security across the organisation.
We are pleased to announce an important enhancement to our email security across the organisation. Currently, we use Mimecast to filter spam, phishing, and malicious emails. Mimecast has served as our email security gateway, blocking threats before they reach your inbox.
On 3 December 2025, the entire organisation will transition to Microsoft Defender for Office 365, which provides integrated protection directly within Microsoft 365. This upgrade will enhance security visibility, simplify management, and improve your overall user experience.
What’s Changing
- All incoming and outgoing emails will now be scanned and filtered by Microsoft Defender instead of Mimecast.
- Quarantined emails (messages flagged as spam or suspicious) will appear in the Microsoft 365 Security Portal, not the Mimecast portal.
What to Expect
- Starting 3 December 2025, all emails will be protected by Microsoft Defender for Office 365.
- You may receive quarantine notification emails from Microsoft Defender.
- If an email is quarantined, you can review and release it safely by following the steps in the attached User Guide – Release Quarantined Emails.
- To block or allow specific senders, please refer to the attached MS Defender User Guide – Block & Allow Senders.
The detailed user guides attached above will help you manage quarantined messages and sender preferences in Microsoft Defender.
Frequently Asked Questions (FAQ)
Q1: What happens to emails currently quarantined in Mimecast?
A: Quarantined emails from Mimecast will no longer be accessible after the transition. If you still need to retrieve these emails after the cutover, please contact the Helpdesk at helpdesk@nswact.uca.org.au or call (02) 8267 4404 for assistance.
Q2: Will I need to install anything on my computer?
A: No installation is required. Microsoft Defender for Office 365 is integrated into Microsoft 365 and works automatically.
Q3: How will I know if an email is quarantined?
A: You will receive a notification email from Microsoft Defender with a link to review and release the message.
Q4: Who do I contact if I have issues or questions?
A: Please contact the Helpdesk via email at helpdesk@nswact.uca.org.au or call us on (02) 8267 4404. Our team is ready to assist you.