Mail Merge exception 800A16DC

when running an automated mail merge, this exception can show up and is somewhat tricky to troubleshoot. It could be one of several issues:

1. Lack of permissions to the mail-merge data
2. The main document is missing the definiation for the mail merge data. (i.e. the document has merge fields but no data document or datasource are defined for that document. This can happen if one inadvertantly removes the mail merge data from the document).
3. The mail merge data source is removed.