The e-mail must have at least one recipient before it can be sent

Hi,

I've created a workflow to send an email once a case has been created however the workflow is created but is stuck with the status "Waiting" with postpone until date of "12/30/9999". It's giving me an error "The e-mail must have at least one recipient before it can be sent" with the error code: -2147218684.

However when I manually click resume on the workflow it works fine as all the contacts the email is sending to have a valid email address.

Why is it giving me an error about the recipient when it can send after I resume to task?

This was working fine when the workflow scope was set to "User" however I've changed it to "Organization" and this has started happening.

Any help would be appreciated.