Have a workflow that just sends an email.

The email goes to two external people and one internal person.  The names are hardcoded so it does not change.

The workflow has been working for months - then for a period of about a month it errored.  The workflow only runs once a week or so.  Now it works again with no changes.

Again, nothing changed with the workflow and the email names are all hardcoded.

In the workflow we simply get the following error:

Both of the recipients are the external users.  The internal user got the email during that time (so they thought the process was fine).  

There is nothing in any server log I can find that matches with those times or gives us anything.  Is there some place I can reference further or is it on the client to verify in their mail logs?