Microsoft Graph: Email Delivery Failure with sendMail API

I’m having an issue sending emails to <_username_> using the Microsoft Graph API. Although I get a 202 Accepted response, I receive a delivery failure notification when I check the user’s mailbox:

Delivery has failed to these recipients or groups:

Your message wasn't delivered because the recipient's email provider rejected it.
Remote Server returned '550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. For more information please go to AS(7171) []'

When I logon to Outlook and manually send an email to <_username_> from <_userId_>'s account, the email is sent successfully.

I’ve also tried adding a SMTP connector, but this did not fix the problem. When I use the SMTP connector to test Outbound Email, the email is received at <_username_>

Does anyone know what could be causing this issue?

The error message suggests that the IP address of the machine sending the email is not authorized to send emails to the recipient’s email provider. You need to ensure that the IP address is added to the allowed list of IP addresses for the recipient’s email provider. You can contact the email provider’s support team for assistance with this.