Windows Live Mail won't send email

My Windows Live Mail (on my desktop Windows 7 computer) won't let me send emails: neither reply nor create a new email.
The error message is "A problem occurred while trying to open this message. A problem has occurred, please try again". Receiving mail works fine.
I checked my account settings, everything looks fine. Moreover, I have a Win8 laptop, with WLM with identical settings, and it works fine on it. The laptop is connected to the same router, so it is NOT an ISP port setting problem.
It's also not a password problem, because receiving mail works fine.
Any idea?

What is the version of your WLM??

Have you tried this process to solve your problem???

1. In your Windows Live Mail application, right click on your email account >> choose Properties from the menu.

2. Choose the Servers tab, select My server requires authentication >> click Settings.

3. Select Use same settings as my incoming mail server, and click OK.

4. Finally, click OK to close the Properties window.

Thank you, Abrienne, but your suggestion is precisely as my WLM is (and has always been) configured from day 1.
As I wrote before, I compared my desktop account configuration to the laptop's, and they are identical. On the laptop it works, on the desktop it doesn't.

The WLM Version: 2012 (Build 16.4.3528.0331)

Thank you, Ranger4, but this won't work.
The Control Panel/Mail can be used for Outlook only, not for WLM.
Control Panel/Mail is currently set to my son's email, which was valid several years ago and is not relevant anymore (this computer belonged to my son several years ago). This proves that Control Panel/Mail has no relevance whatsoever to my WLM account, and clicking "repair" in it won't fix my WLM.

No problem at all.