Unable to send message;Reason:554 5.1.8 Sender Address Blocked

Unable to send message;Reason:554 5.1.8 Sender Address Blocked

Hi Team,

I'm unable to send messages to my potential clients and the message " Unable to send message;Reason:554 5.1.8 Sender Address Blocked" appears. Reading from earlier posts, it seems that my account has been blocked.
There is a supposed limit to the outgoing messages however I was unaware and also no pop up was displayed informing that limit has been reached.

May I request you to please unblock my account and confirm on the daily limit, both in terms of message size and number of messages.

Thanks,