Are emails from OnSIP getting blocked or delayed in receipt to Mimecast?  This is what a few of our customers with the same issue did and provided to OnSIP to share with our customers:

From mimecast to OnSIP customer...."To prevent this from occurring, you'll want to go ahead and create a message pass through policy. This can be found under:
Administration > Gateway > Policies > Message Pass through
https://community.mimecast.com/docs/DOC-1421

Use the from address admin@onsip.com for the messages to be delivered successfully. For best results, use the domain onsip.com in the Specifically field for domain. We occasionally change or modify email addresses.

After doing so, have the service resend the message and see if its getting caught...."

In addition, they did the following for the policy above:
"....The problem was that I was using the wrong email address in the Mimecast policy. It should have been admin@onsip.com rather than do-not-reply@onsip.com. Now we are receiving those emails correctly..."

November 2017