Microsoft OAuth 2.0 Authentication Failure - token_url Bad Request Error - azure

Question

I want to ensure that my OAuth 2.0 authentication credentials. So, I have client_id, client_secret, tenant_id, scope, token_url and auth_url. I am using third party app in order to send a email but, I get error in OAuth 2.0 part as Bad Request Error for token_url.
In this case, when grant_type='client_credentials', how can I fix token_url domains or subdomains?
-> token_url like that https://login.microsoftonline.com/<tenant_id>/oauth2/v2.0/token
If you are dominate the this subject, Could you please mention about OAuth 2.0 process?
I tried to changed grant_type parameter as client_credentials. Default was 'refresh_token'.
In this case, it returned like that error:
(530, b'5.7.57 Client not authenticated to send mail. Error: 535 5.7.3 Authentication unsuccessful [ZR2P278CA0041.CHEP278.PROD.OUTLOOK.COM 2023-02-14T08:10:26.343Z 08DB0DEF6EA5D39B]', 'myemail#myemail.com')
2023-02-14 11:10:26,378 INFO [decorators] [send_email] StatusMessage: An error occurred while sending the email: (530, b'5.7.57 Client not authenticated to send mail. Error: 535 5.7.3 Authentication unsuccessful [ZR2P278CA0041.CHEP278.PROD.OUTLOOK.COM 2023-02-14T08:10:26.343Z 08DB0DEF6EA5D39B]', 'myemail#myemail.com')
2023-02-14 11:10:26,379 INFO [decorators] [send_email] StatusMessage: Done with sending email...
So, what is the main problem in this case?
Thank you for your attention,

Answer 1

I tried to reproduce the same in my environment and got the results as below: 
I created an Azure AD Application and granted admin consent to Mail.Send API permission:
  
Based on your requirement you can make use of Client Credential Flow or Authorization Code Flow.
If you want user interaction then make use of Authorization Code Flow.
If you want to send mail as Application then make use of Client Credential Flow. 
I generated the access token via Client Credential Flow by using parameters like below:
https://login.microsoftonline.com/TenantID/oauth2/v2.0/token
client_id:ClientID
client_secret:ClientSecret
scope:https://graph.microsoft.com/.default
grant_type:client_credentials
 
 To send the mail, I used below query:
https://graph.microsoft.com/v1.0/users/FromAddress/sendMail 
{
"message": {
"subject": "Test mail",
"body": {
"contentType": "Text",
"content": "Test"
},
"toRecipients": [
{
"emailAddress": {
"address": "****"
}
}
],
"ccRecipients": [
{
"emailAddress": {
"address": "****"
}
}
]
},
"saveToSentItems": "false"
}
 
 Reference: 
user: sendMail - Microsoft Graph v1.0 | Microsoft Learn