Unable to send message;Reason:554 5.5.3 Mail Policy violation : Too many recipients.

Unable to send message;Reason:554 5.5.3 Mail Policy violation : Too many recipients.

I have two paid domains and hosting from Zoho.  I have permission from the district to send out fundraising proposals to all the public schools in the area.  I'm sending about 100 something emails at one time, but it's not letting me send them.  Please, resolve this for me.