Outlook Express 6.0 - Trouble sending emails with attachments

You have apparent dbx file corruption, but easily fixed.

Move most of your messages out of the Inbox and then create new Outbox and Sent Items folders after you move any messages you wish to save to a local folder you create.

Tools | Options | Maintenance | Store Folder will reveal the location of your Outlook Express files. Write the location down and navigate to it in Windows Explorer or, copy and paste it into Start | Run.

In WinXP, Win2K & Win2K3, the OE user files (DBX and WAB) are by default marked as hidden. To view these files in Windows Explorer, you must enable Show Hidden Files and Folders under Start | Control Panel | Folder Options Icon | View, or in Windows Explorer | Tools | Folder Options | View.

With OE closed, find the DBX files for the Outbox & Sent Items and delete them.  New ones will be created automatically when you open OE.

After you are done, follow up by compacting your folders manually while working *offline* and do it often.

Click on Outlook Express at the top of the folder tree so no folders are open. Then: File | Work Offline (or double click Working Online in the Status Bar). File | Folder | Compact all folders. Don't touch anything until the compacting is completed.

General precautions for Outlook Express:

Do not archive mail in the Inbox or Sent Items. Create your own user defined folders and move the messages you wish to save to them. Empty Deleted Items folder daily. Although dbx files have a theoretical capacity of 2GB, I recommend about a 300MB max for less chance of corruption.

Information about the maximum file size of the .dbx files that are used by Outlook Express:
http://support.microsoft.com/?kbid=903095

Turn off e-mail scanning in your anti-virus program. It is a redundant layer of protection that eats up CPUs, slows down sending and receiving, and causes a multitude of problems such as time-outs, account setting changes and has even been responsible for loss of messages. Your up-to-date A/V program will continue to protect you sufficiently. For more, see:
http://www.oehelp.com/OETips.aspx#3 

Why you don't need your anti-virus to scan your email
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm

Note that for some AV programs, it may be necessary to uninstall the program and reinstall in Custom Mode and uncheck e-mail scanning when the option arises. 

Compact often as specified above.

And backup often.

Outlook Express Quick Backup (OEQB Freeware)
http://www.oehelp.com/OEBackup/Default.aspx