Sending failed;Reason:554 5.1.8 Sender Address Blocked

Sending failed;Reason:554 5.1.8 Sender Address Blocked

Hello

I received this message when trying to send emails today

Apparently the reason is 

Dear User,

Your Mail Outgoing blocked. Reason :Daily Mail Count Exceeds Limit  [Mail Count=255]

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This is an automated message, please do not reply to this mail - Zoho Mail Administration

my email is vsmokovitis@zoho.com

I am not sure what i did wrong, please explain so that it doesnt happen again.

And please unblock my email address 

Thanks a lot