Your Outgoing has been blocked and you will be unable to send emails

Your Outgoing has been blocked and you will be unable to send emails

We've had a client call us saying they had a warning message stating "Your Outgoing has been blocked and you will be unable to send emails".

I've checked the control panel for the domain and there's this message for the account:

User :  geral@paulo-aguiar.com  
Details :Mail Outgoing blocked. Reason :Mail Recipient count exceeds limit [Recipients=243] 


What are exactly the limitations of each email address on your free plan?