Outgoing email blocked for one user

Outgoing email blocked for one user

We are using one email ID " registration@ezyremit.co" for sending email about the the application registration and confirmation. from last 3-4 days the email id's outgoing mails has been stopped automatically and when i am trying to enable it from admin console, i get the error message that" this "operation is permitted for the paid users only" . My organization is under 25 users to thats the reason we have chosen the free tier. 

Although the allocation is pretty critical for us as the user registration is dependent on the email ID. can you please help me enable it.

Thanks

-Abhijeet