Introduction

You have setup the exalate notifications in your node, but no notifications are sent.  

Problem

Currently mimecast (and maybe other mailservers) are bouncing mails from mail.com due to an unknown reason.
Please make sure that mails from exalate.com are not being bounced.

Resolution

Configure 'the permitted senders' option on your mailserver.
In the case of mimecast - check following document
https://community.mimecast.com/s/article/Configuring-Permitted-Senders-Policies-1067720131