Unable to access the shared mail box through Graph API

we are implementing an console application which will Read messages from the shared mail box shared@****.com through Graph API. we have the delegate permissions like User.ReadWrite.All, Mail.ReadWrite.Shared, Mail.ReadWrite. we are able to fetch authentication token by login with service account(serviceac@***.com). but while we trying to access shared mail box. it was throwing an error as

 

"Code: ErrorItemNotFound\r\nMessage: The specified object was not found in the store., The process failed to get the correct properties"

 

The code which i have used is here.

 

await graphServiceClient .Users["shared@****.com"].MailFolders.Inbox.Messages.Request() .GetAsync();

 

please help me to resolve this issue.