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It would be helpful to know exactly which version of Windows Live Mail (WLM) you are running, there are like 4 different versions (WLM2009, WLM2010, WLM2011, & WLM2012). There have been some recent updates and changes to older versions of WLM from Microsoft that may affect your ability to use WLM; see the Microsoft article here: Live Mail 2012
Furthermore, it would also help us to know which version of Windows you are running your WLM on as well (for example, WindowsVISTA,Windows7, Windows8, Windows8.1, Windows8.1.1, Windows10)?
Generally, a message stuck in the Sent Mail folder indicates a corrupted message, or a problem with your Account settings (who is your Internet provider, please??; i.e., Cox, Verizon, Charter, etc.). Usually, it's best to start with deleting the message which you've already tried.
The next thing is to run the Mailbox maintenance that comes built-in with WLM. Here's a link to that: ** this is from the Microsoft Community Forum**
Bigbear, I found the problem & was able to delete the message, was easy when I figured it out! I have Windows 7. I got a message from Michael Fara regarding this forum. I was having a hard time with understanding it. I am a senior so it is not easy for me. He is going to look into it so maybe it will be more simple in the future. Thank you for responding.