There are many ways to generate url to open email. But the simplest way is to build the URL in the following pattern

https://outlook.office.com/owa/?ItemID=[MESSAGE_ID]&viewmodel=ReadMessageItem&path=&exvsurl=1

In this case the message ID is the ID of the email.

Other References

https://superuser.com/questions/1503128/link-to-microsoft-outlook-365-webmail-email