After viewing the Outbox, messages cannot send

Symptoms:
The last week my customer experience problem. When users are offline and send email, they are kept in outbox. when they connect to the network the message in outbox are sent.
But, when users viewing email in outbox, email cannot send when connected back to the network.

Environment:
mac-book-air with OS windows 7, mobile, and WiFi only, Outlook 2010

i'am also has been try to solve with this site  http://www.slipstick.com/problems/after-viewing-outlooks-outbox-the-messages-in-it-wont-send/, but the problem hasn't solve.

Can you help me fix this problem?

Thanks and regards,
Panji Arya Sanjaya