Outgoing Mail Logs
When users have issues with outgoing emails, they can start troubleshooting by navigating to the Configuration Application, Email Logs Module, Outgoing Mail Logs. You may get the following error when you click on the "Outgoing Mail Logs":
Try adding a filter to the Screen so when the Outgoing Mail Screen is displayed now it will be the Filter Overlay shown rather than any data. This will give the user the ability to start filtering through data to identify the record that is causing the error.
Once the record is identified, you can now identify the process (by matching the time of the email to the processes being ran) to get the calculation log.
In this case it turns out that there were carriage returns in the entries for email addresses rather than commas which messed up the record.