People whom I send encrypted emails with attachments cannot open them

Hi @Glenn Irvine ,

Did you send emails using Outlook desktop client? If so, are there any error prompts when this issue occurs? To further confirm your issue, it is suggested for you to send this email to multiple users at the same time and check that if other users can open the attachments normally.

To confirm if your issue is related to your Outlook client, please check this email from the recipient‘s webmail and see if the attachment can be successfully open.

From the perspective of the client, as I know, we could encrypt emails in several different ways such as S/MIME encryption and Microsoft 365 Message Encryption. So how did you encrypt your sent emails?

In order to better solve your issue, please provide us with more information related to your issue. Any update, please feel free to post back.