Unable to send email message on behalf of another O365 outlook account - microsoft-graph-api

Question

For some reason the 'Send on Behalf' feature is failing for me. Graph always returns an ErrorSendAsDenied (when From and Sender field values are different).
The accounts are verified to have the correct configuration for sending on behalf of each other. I couldn't make it work after spending more than three days on it.
Following permissions are requested during authentication:
openid
email
profile
offline_access
User.Read
Mail.Read
Mail.ReadWrite
Mail.Read.Shared
Mail.ReadWrite.Shared
Mail.Send
Mail.Send.Shared
For the example below, the user Gregory Test wants to send messages on behalf of Sem Test. A valid authorization access token (authenticated by Gregory Test) is used for sending the email, however email delivery ALWAYS fails.
The sample submitted JSON message is:
{
"Message":{
"Subject":"Trying to send message on behalf",
"Body":{
"ContentType":"html",
"Content":" email html content "
},
"From":{
"EmailAddress":{
"Address":"sem.test#contoso.com",
"Name":"Sem Test"
}
},
"Sender":{
"EmailAddress":{
"Address":"gregory.test#contoso.com",
"Name":"Gregory Test"
}
},
"ToRecipients":[
{
"EmailAddress":{
"Address":"semtestestestes#gmail.com",
"Name":"Sem Shehovtsov"
}
}
]
}
}
If the Sender and From are set to the same value, it is working fine. The Sender mailbox is configured to allow sending messages on behalf of other users.
Any ideas?

Answer 1

You don't need to set the Sender property, Graph will handle this automatically for you. From the documentation:
In order to send from another user, set the from property on the message sent to the email address of the user to send from. You don't need to set the sender property - Microsoft Graph will set it appropriately, based on the mailbox permissions granted to the user who has signed in.
Try this payload instead:
{
"message":{
"subject": "Trying to send message on behalf",
"body":{
"contentType":"html",
"content":" email html content "
},
"toRecipients":[
{
"emailAddress":{
"address":"semtestestestes#gmail.com",
"name":"Sem Shehovtsov"
}
}
],
"from":{
"emailAddress":{
"address":"sem.test#contoso.com",
"name":"Sem Test"
}
},
}
}
Also, you don't need to request both Read and ReadWrite scopes. The ReadWrite scopes overlap with the Read scopes.