This can happen if your hosting account has been automatically detected for sending too many emails.  This can happen if you send newsletters to lots of email addresses, or if you have an unprotected contact form that is spammed by a bot or a compromise to a PHP script that allowed spam to be sent, or many other reasons.

The solution is to contact support at:

https://support.ifastnet.com

And we can provide you a log of the emails sent to establish the best way to resolve the issue and get your email un-suspended.