Hi Vikas,
Currently, it’s not feasible to check how many emails a user has sent or get a reports about it.
In my opinion, it’s a very practical feature for the Exchange online service. I will written a summary and deliver it to our higher level team.
In addition, I suggest you go to support.microsoft.com/.../survey.aspx to submit feedback about this.
Thanks for your time.
Regards,
David Zhang