Opera Mail for win7
I just installed Opera mail on both my Win 8 laptop and Win 7 desktop. No problems with laptop.
My desktop is currently running Opera 12.16 and I needed to install mail before update because the mail has been removed from new opera. It imported all my old mail and setting from Opera browser but it wont let me enable HTML. All my emails are stretched over the width of the screen and in plain text.
Settings 'wrench' does not display same options as it does in Win 8, and there is no option to uncheck "prefer plain text"
While Opera Mail is closed, rename its operaprefs.ini (see Help -> about Opera mail for the location) and see if that fixes it. Delete the "toolbar" and "menu" folders too.
It imported all my old mail and setting from Opera browser
What import wizard did you use in Opera Mail? There's the "Import from Opera" one that just imports account settings and import messages and there's the wizard about it that imports everything (as if you copied the mail folder, operaprefs.ini, contacts.adr and wand.dat over from Opera 12 to Opera mail yourself).
You might want to download the Opera Mail installer, launch it, click "options", set "install path" to your current Opera Mail so that the button says "upgrade" and choose to upgrade. That will fix messed-up Opera Mail program files if any.