Application permission: Send As email

[IgnacioMaestro]

In an app with Application permissions and credentials flow, I have Mail.Send.Shared permission. I want to send an email with the Send As mode. I mean, I have a shared mail box and I want to send an email from this mail box and I have another user with the permission to do that. Is it possible with python-0365?
I have tried that way:

    credentials = (client_id, client_secret)
    account = Account(credentials=credentials, auth_flow_type='credentials', tenant_id=tenant_id,
                      main_resource='user@my_company.com')
    if not account.is_authenticated:
        ok_auth = account.authenticate()
        if ok_auth:
            print('Authenticated!')
    if account.is_authenticated:
        message: Message = account.new_message()
        message.sender.address = 'shared_mailbox@my_company.com'
        message.to.add('another_user@my_compnay.com')
        message.subject = 'example subject'
        message.body = 'example message'
        message.send()

And I'm getting this error:
Client Error: 401 Client Error: Unauthorized for url: https://graph.microsoft.com/v1.0/users/user@my_company.com/sendMail | Error Message: The token contains no permissions, or permissions can not be understood.

[alejcas]

Did you set application permissions or delegated?
Did the administrator approve your application permissions?

[IgnacioMaestro]

Yes, I have an app created with application permissions and the administrator has approved them.

[alejcas]

Then you don’t have the permissions required to send email as “ user@my_company.com”

I mean you don’t have access to this user email.