These additional details reference an export definition. The email process is referencing the export and does not see the object it is looking for. So in this case a user built an email with a merge field that no longer exists in the export definition used to create the email. Adding the field back to the export definition or re-creating the email resolves this issue.
On one case the error was caused by having the one-to-one column headers checkbox enabled in the CRM Export Definition.
If this does not resolve your issue Chat with Support and reference this article.