Mail stuck in Outbox in Microsoft Outlook Express
Wednesday, 24 October 2007
If you are having trouble with mail being stuck in your Outbox in Microsoft Outlook Express then the following Microsoft technote may help:
Microsoft: How to troubleshoot e-mail messages that are stuck in your Outbox
One of the most common fixes it mentions is to exit outlook express, find and delete the outbox.dbx file then restart outlook – however, this will clear the contents of your outbox !

Jason Slater is an independent technologist and blogger.
Thanks for posting this KB article URL, includes all that I collected from several forums, and might just contain the right solution.
There is good software which work with mails-.dbx viewer,it helped me some times ago,as far as i know it is free,utiltiy will analyze these corresponding files of dbx format and extract messages, if it is possible,program attempts to retrieve messages and save restored emails to your HDD or any other removable media,also with other mailbox recovery solutions and make sure, that this program works better.