As long as you make backups, it's OK to use.
I only use registry cleaners after I uninstall software and I only remove entries they suggest, that look like they might be related to that software.
If you think this is related with your last Registry mechanic session, apply the backup that was made.
The problem with OE is that the mail windows never open maximized.
You can make them open full-screen, but never truly maximized.
What you would have to do is find the window, which is probably too small to display or outside the desktop view.
Then drag it to the proper location and size and then close it using the "Close button"
This behavior is described here:http://support.micro...com/kb/q190277/
If you look in the backups of Regsitry Mechanic you may find keys that look like this:
If we knew what <number> we needed we would be halfway there.