Outlook Classic cannot open protected emails

If Outlook Classic cannot open protected emails at DumpsCafe or within your Microsoft 365 environment, the issue is usually related to Information Rights Management (IRM), Microsoft Purview Message Encryption, or a recent Outlook update.

For the desktop, I'd check the following

VPN

Antivirus

Windows updates

Office updates

Any 3rd party firewalls

I noticed you haven't removed the credentials, that sometimes does help too

Facing trouble while opening protected emails in your Outlook Classic. It happens with every 3rd users, this issue arises due to some unavoidable reasons. I am sharing a list of reasons below, in which any one of the reasons occurs when you are trying to open protected emails. So, after reading these reasons, you might find the exectly happen reasons with you.

• Wrong account signed into Outlook
• Missing Microsoft 365
• Outdated Outlook version
• Corrupted Office credentials & authentication cache
• Unsupported Outlook version
• Internet issue with Microsoft services
• Corrupted Outlook profile

These are the major once which is mostly happen to everyone. So, before making any decision regarding a solution, do checks first.

Since the same message opens in OWA and your tenant-side IRM checks pass, I’d treat this as a Classic Outlook identity or license-acquisition issue, not an Exchange transport problem.

First, in Outlook go to File > Office Account and confirm the signed-in, activated Microsoft 365 account is the mailbox that received the protected message. Sign out of Office apps, close them, then sign back in and retest. Also try the mailbox on another managed PC or a fresh Windows user profile; that quickly separates an account or Conditional Access issue from a damaged local identity cache.

If the messages come from another tenant, review Entra cross-tenant and Conditional Access policies for Microsoft Rights Management Services (app ID 00000012-0000-0000-c000-000000000000). Microsoft documents that Outlook desktop must reach the AIP/RMS endpoint, while OWA can still work because decryption happens in the service. Check proxy or TLS-inspection logs too. If it fails only on one PC, capture the Office build and sign-in logs for Microsoft support before deleting any RMS or identity caches.