Hi @Md. Mahfuzur Rahman ,
In order to narrow down the issue, I wonder to confirm the following points with you:
- Are you sending mail using the Outlook client or OWA?
- When you send messages to your Outlook account, do you get a similar error?
- Is the certificate installed in the client that you sending the message?
Here are two official Microsoft guides on how to send encrypted emails from outlook client or OWA, you can refer to for more information:
Encrypt email messages - Microsoft Support
Encrypt messages by using S/MIME in Outlook Web App - Microsoft Support
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