Hello Shreyas Rastogi,
Thank you for reaching out to Microsoft Support!
According to the document Metered APIs and services in Microsoft Graph, there is no billing for sending emails using the Graph API.
So, there is no cost for sending mail in bulk, but there are frequency limits for endpoints that may be limited when you send a lot of requests in a very short period of time.
Using the Graph API to send email refer to the documentation, and you can try it with generic email addresses.
Hope this helps.
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