@Ahmed Sh Thanks for posting in our Q&A.
Honestly, I'm not familiar with exchange. Based on my research, here is an article about using S/MIME to encrypt email.
https://learn.microsoft.com/en-us/mem/intune/protect/certificates-s-mime-encryption-sign
Hope it is what you want.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.