Unable to send mail - Unable to send message;Reason:554 Email Outgoing Blocked

Unable to send mail - Unable to send message;Reason:554 Email Outgoing Blocked

Hello--

I'm getting the message "Unable to send message;Reason:554 Email Outgoing Blocked" when I try to send a message. I understand you monitor for suspicious activity, but I have only sent about 10 emails since I opened my account last week, so I'm a little confused about what the issue might me.  Please advise.

Thanks--

Donald