Problem : When sending email the message is discarded and the error message 

'Domain $domain.com has an outgoing mail suspension. Message discarded.'

is displayed. 

Solution : This happens when bulk mail / mass mailing is detected for your hosting account.  The system will then automatically disable email sending. 

Mass mailing causes our servers ip addresses to get on blacklists, and thus causes other customers to have issues sending email, therefore we have to act to preserve our networks reputation

These mass emails are normally caused by hacked websites, or compromised email accounts.

Your next stop is to create a support ticket at https://support.ifastnet.com/new_ticket.php, requesting a copy of your mail logs, so you can identify the cause.