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

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

Hello,

I'm using one of the email addresses for sending out automated application notifications. This is for internal use, so there is a limited recipient list (these notifications are sent to no more than 20 addresses). But depending on the events that trigger the specific message, in a given time period there may be more than usual number of outgoing emails.

I had noticed therefore that sometimes I would get a undelivered mail message or temporary block, which will be resolved soon. But currently my address seems to be blocked permanently since Sept 27. If I try to send a message manually from the web client I get this address blocked status.

I would appreciate much if you could help me understand what is going on.

Thank you,
Zurab