Hi please provide more detail
If by secure email you mean DKIM/DMARC, see here; https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/email-authentication-dkim-configure?view=o365-worldwide and there https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/email-authentication-dmarc-configure?view=o365-worldwide
If you mean secure for an end user view, please see https://support.microsoft.com/en-us/office/set-up-your-microsoft-365-sign-in-for-multi-factor-authentication-ace1d096-61e5-449b-a875-58eb3d74de14