Hi @EnterpriseArchitect ,
Based on my knowledge, the full access permission only allow the delegate to open the mailbox, and view, add and remove the contents of the mailbox, you cannot open Microsoft 365 Defender portal for CEO without sharing the password.
But you could ask the Admin to enable end-user spam notifications in anti-spam policies, the original message recipients can review quarantined phishing messages (not high confidence phishing messages) directly from these notifications, so that you could view these quarantined emails in the CEO's mailbox.
Note: Messages that were quarantined as high confidence phishing, malware, or by mail flow rules (also known as transport rules) are only available to admins.
For more details: Use user spam notifications to release and report quarantined messages
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