This issue is severe. The Outlook PST parser extracts a string for the body of every email and passes that string to the EmbeddedDocumentExtractor.
However, no content type is set on the Metadata object passed to the extractor. Therefore, if for example, the body of the email starts with the string "From John Smith." (for example, when an email was forwarded), then body of the email is detected as application/mbox and parsed as though it were an mbox file.
I think the immediate fix for this issue is to force the type of the email to text/plain and for it to be parsed as such.