Hi @Patidar, Jaydeep,
You could use lookupUserId to get user item. The Look Id is the Id in the SharePoint Hidden List named "User Information List", you can get the list guid using the request:
https://graph.microsoft.com/v1.0/sites/site-id/lists?$filter=DisplayName eq 'User Information List'
Then use the list guid in the endpoint below to get the detail email address:
https://graph.microsoft.com/v1.0/sites/site-id/lists/UserInformationListId/items/lookupid
Here is a similiar thread for your reference:
How to get user from User field LookupId
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