Question that I haven’t seen asked (or answered) yet. What program do you use for emails - if it is a standard Microsoft email client (Outlook, outlook Express, Microsoft Mail (old program abandoned years ago), Microsoft Mail (unrelated new program with a different mail) then all the emails are probably balled up in a single .pst file - deleting them will likely be an all-or-nothing exercise. If anything else, there are probably copies of them stored away on the internet somewhere that will never be forever gone. (And, indeed, there are, in any event, probably copies on an NSA controlled server somewhere; maybe here, maybe in England.) So you might as well give up on the thought that you can really do anything about emails except put investigators to some effort.
As for other files, there are likely solutions. I knew of one federal agency that had a commercial quality drill press with a hardened bit that they used for no longer needed hard drives. It seemed reasonably effective.
I’m a big believer in deleting old files of every thing I have no need of. What I want to keep I back up to an off line drive. I use OE and I recognize that it’s on the servers but, I just want to keep my online system free of identifiable info as a preventative measure to limit hackability.
“Just cause you’re paranoid doesn’t mean...”
Privacy is an illusion in todays world but, I like to keep things minimized.
I purchased a subscription to the Office 365 Business Email service. I pay about $4/mailbox/month but I get the same Enterprise quality service that larger companies get. I use my personal domain name with it. For as long as I’ve had it, several years, I haven’t experienced any outages.
I prefer paying a little to have my own private email rather than rely on free services like GMail or Yahoo.