Authentication failed 554 5.1.8

Authentication failed 554 5.1.8

Hello, 

I've still got no access to send email. I verified that I had no bounced addresses - I sent to a very small list of people (approx 12) and sent small emails (less than a MB) - please reactivate my account ASAP and explain why I was blocked from sending. 

Since signing up I've sent this same list of people maybe a total of 10 emails - What would cause my account to be blocked from sending?

Is there a way to avoid this happening in the future? 

I am looking for a service that will be reliable for our small business needs, but this seems to be a big problem. 

Please advise.

twhidbey@r3communityservices.org