There used to be a patch program somewhere in the knowledge base for Printshop Mail that did this automatically, but I've just been updating the registry.
***Do not do this if you are not comfortable editing your computer's registry-I'm just posting how I do it on Windows XP
Run
regedit
go to:
Hkey_current_user
Software
Printshop Mail
Printshop Mail 7.0 ( or your current version)
Settings
In the right hand pane, one of the items is CropmarksLineWidth. Double click and change the value. I have mine set on 3, which is a little heavy, so 2 may work for you.