Assuming the task is configured correctly, I have run into this or similar before. In my case, the target email had incidentally been added to the SnapLogic email relay suppression list, and that itself was blocking the failure notifications from being delivered. In troubleshooting, I found the task was able to send to other email addresses, but not the intended one.
This may not be your situation, but that is my recommended troubleshooting starting point. Note, at the time, the suppression list was case sensitive and I was able to work around the issue by changing the email address prefix casing. Ultimately, however, I opened a support ticket with SnapLogic to have the email address removed from the relay suppression list.
