Gary is absolutely right about plug ins. But there is a third-party add on
that allows reformatting, it also removes attachments and allows send again.
I have used it for some time. It's not free, but if you want the features,
and want to stay with Windows Mail as I did, then Google Windows Mail
QuickTools. They do just as they say.
There is also a repair tool for Windows Mail, that solves many of the
problems like stuck mail, and the tendency of Windows Mail to accumulate 0
byte files. It is freeware. Just Google for WMutil.exe. It is tiny, and
doesn't need to install. It will only run if Windows Mail is not running.
Good luck
Sorry, there are no plug-ins for Windows Mail, nor is there any likelihood
that there ever will be. Microsoft has stopped all development on Windows
Mail in favor of the successor program, Windows Live Mail.
As of yet, Windows Live Mail does not have any plug-ins either, but
at least there is a chance that will happen in the future.