From: DeskNow Knowledge Base

552 Too many recipients

If you're getting this error while sending an email from a mail client (Outlook, Eudora, etc), it means that your email contains more recipients (addresses in the To, Cc, or Bcc fields) than allowed by your DeskNow server.

The maximum number of recipients allowed is set by the admin user in the "Administration/Mail/SMTP Incoming/Traffic control/Maximum number of recipients per SMTP connection" setting.