Last September, we announced new capabilities in Office 365 Message Encryption that enable users to seamlessly collaborate on protected emails with anyone. This release included Do Not Forward an out...
Updated May 11, 2021
Version 11.0
Blog Post
There are a couple of different issues being mentioned in this discussion, but I (and at least a couple of others) posted previously that there is a problem not just with Outlook desktop, and not just with attachments, when sending encrypted messages to external Office 365 tenants.
I have had an issue with recipients at two different agencies who both use Office 365. When I send them an encrypted only message, they are unable to open it at all - even if there is no attachment.
They have tried this with both Outlook 2013 and 2016 desktop clients,
They have also tried this with Outlook web and were unable to open the emails.
The message they receive when attempting to access the message is:
"The message you tried to open is protected with Information Rights Management and can only be opened using Outlook. Download a free trial of Microsoft Outlook."
Sending to anyone else who is not an Office 365 tenant works fine.
I haven't tested this in the last couple of weeks, but haven't seen any communication that this is fixed.