Are there any restrictions on the number of recipients allowed for a single email?

Yes. In order to help ensure our systems are not used for the sending of spam emails, we restrict the maximum number of recipients in any one email to 100. Please ensure that any emails you send contain less than 100 recipients. 

Upon receiving an email with greater than 100 recipients, our system will send it to the first 100 email addresses and then reject it with a failure notice to the sender. Therefore, if you attempt to resend an email which has failed because it contained more than 100 recipients, please ensure to remove the first 100 original recipients before you send it again to avoid sending duplicate messages.

If you wish to send out regular mailings to large numbers of recipients, please see this article for more information.