Found a solution:
From user "magjis" at
Droid X Forum
I experienced the exact same problem on my Droid X. I clicked yes to send a return receipt on a text message containing an image. The real problem appears to be that the default "Text Messaging" app does not show you a failed message queue, but will continually try to resend them. The good news is that Handcent shows you this queue and allows you to delete these messages. You do not need to continue to use Handcent, just use it to delete undelivered messages (I like the appearance and feel of the default app). Make sure to turn off Handcent's notifications if you plan to keep it installed as a deletion utility.
Solution:
1. Install Handcent SMS from Android Marketplace
2. Launch Handcent SMS
3. Press Menu Button >> More + >> Undelivered messages
4. Here you can delete undelivered conversations and end your "messages not sent" woes!