Dear ZOHO team

We need to send regular mail to our customer using noreply@ipayremit.com and otp@ipayremit.com using our .NET code. We have even registered the APP Name Ipayremitpanel and were successful in sending the mail. However now we are getting 

Mailbox unavailable. The server response was: 5.4.6 Unusual sending activity detected.

Since our business very much depends on sending email to our clients (we are in money service business). Can you please advise us what we should do. If sendng automated email is feature not provided by ZOHO then is their some product or service you provide that we can use to send such email

I look forward to your kind respobse