Outlook and Outlook Express often have difficulty in sending email with large attachments; the email seems to get stuck in the outbox but is actually being sent to the recipient and the recipient gets multiple (for example 20) copies of the same email. There are a couple of common reasons for this:
Anti-virus / Anti-malware Software
The most common reason for many copies of the same email being sent can be traced back to your mailware or virus scanning tools attempting to scan outgoing messages. This tends to interfere with the proper operation of the mail program, such as Outlook. Outlook can sometimes think "well, it didn't send so I'll send it again ... and again". In such cases we recommend disabling the scanning of outgoing emails in your Anti-virus / Anti-mailware software. Once you have done this, you may need to save your work and reboot your computer for the changes to take effect.
Outlook Express mailbox limits
Outlook Express has a 2GB filesize limit for its mailboxes. Sometimes the Sent Items folder can exceed this limit when emails with large attachments are being sent. Smaller file-size emails seem to make it through without causing the mailbox size on your computer to go over the limit.
If this is the case, you could create a new folder in Outlook Express (call it something like Sent Items - Archived) and move all of your sent messages into the folder. You will then have an archive of all your sent items if you need to refer to them later, and you would have freed up space in the Sent Items folder.