Sending Failed: address blocked: ??

Sending Failed: address blocked: ??

Hmm...I just signed up for Zoho services for my entire school.  I have many teachers to send messages to and need replies from.

Sent one mass email to them, then the next day I get this error...my account is blocked. 

?

What are the rules applying to mass emailing?  I signed up for Zoho thinking it would ease our ability to communicate effortlessly via an online collaboration tool.  But if we cannot send out emails/messages to the entire group it kind of defeats the whole purpose, no?

So, please help!  Let me know what I can do to help my staff...if you unblock my email, great!  But they will have to send mass emails every now and then.  I wouldn't want you wonderful people at Zoho to have to unblock each person individually.

Thanks,
-db