I've seen this intermittently with our mail server - more when we ran Notes than since moving to Exchange - and I think it's a timing issue. If the mailserver is busy, then requests from DataPump sometimes just hang with the results you describe. If you have detailed logs, it might be worth talking to the admins of the mailserver to see what they see going on their side.
Reducing the number of large attachments being sent by mail is generally a good thing, anyway, so we've moved what we can to SharePoint. But we've still got some users who think that having an Excel file emailed to them every week is "business intelligence"...
All the best,