Securing your Microsoft 365 email environment is critical to protect sensitive data, prevent unauthorized access, and defend against various cyber threats. Here are steps you can take to properly secure your Office 365 email environment:

1. Multi-Factor Authentication (MFA): Enable MFA for all user accounts. This adds an extra layer of security by requiring users to provide multiple forms of verification before accessing their accounts.
2. Strong Password Policies: Enforce strong password policies that require complex passwords and regular password changes. Discourage password sharing and reuse.
3. Conditional Access Policies: Implement conditional access policies to control access based on factors like location, device, and user roles. This helps prevent unauthorized access.
4. Data Loss Prevention (DLP): Use DLP policies to identify and prevent the sharing of sensitive information through email. Set up rules to detect and block sensitive data from leaving the organization.
5. Email Encryption: Implement email encryption to protect sensitive information sent via email. Office 365 provides encryption options for both internal and external communications.
6. Anti-Phishing Measures: Enable anti-phishing features, such as Exchange Online Protection (EOP) and Advanced Threat Protection (ATP), to detect and block phishing emails and malicious attachments.
7. Anti-Malware Protection: Utilize built-in anti-malware solutions to scan and block malicious attachments and links in emails.
8. Secure Email Gateways: Consider using third-party secure email gateways to provide additional layers of protection against spam, phishing, and malware.
9. Email Quarantine and Review: Set up email quarantine for suspicious emails. Regularly review quarantined items to ensure false positives are released and genuine threats are caught.
10. Regular Software Updates: Keep your Office 365 environment up to date with the latest security patches and updates.
11. User Training and Awareness: Educate users about email security best practices, including recognizing phishing attempts, avoiding suspicious links, and reporting unusual activities.
12. Mobile Device Management (MDM): Implement MDM policies to control and secure access from mobile devices. Ensure devices are enrolled and compliant with security policies.
13. App Permissions and OAuth: Review and manage third-party app permissions and OAuth settings to prevent unauthorized access to user accounts.
14. Audit Logging and Monitoring: Enable audit logging and monitoring to track user activities and detect potential security breaches.
15. Secure Sharing and Collaboration: Configure OneDrive for Business and SharePoint Online with appropriate sharing settings to ensure controlled and secure collaboration.
16. Advanced Threat Protection (ATP): Consider using Office 365 ATP features like Safe Links and Safe Attachments to provide additional protection against malicious links and attachments.
17. Azure Active Directory (AAD) Security: Secure your AAD environment to prevent unauthorized access to Office 365 resources and user accounts.
18. Regular Security Assessments: Conduct regular security assessments, penetration testing, and vulnerability assessments to identify and address potential weaknesses.
19. Incident Response Plan: Develop an incident response plan that outlines steps to take in case of a security breach or data compromise.
20. Third-Party Security Tools: Consider integrating third-party security solutions to enhance your Office 365 security posture.

Remember that cybersecurity is an ongoing process, and threats are constantly evolving. Regularly reassess and update your security measures to adapt to new risks and vulnerabilities. Consulting with cybersecurity professionals or experts can also help you implement a comprehensive and tailored security strategy for your Office 365 environment.