The file system is the simplest way.
To attempt to resend the emails, move the files in ~\ColdFusion2016\cfusion\Mail\Undelivr to ~\ColdFusion2016\cfusion\Mail\Spool.
If you just don't care about those email files then simply delete them.
I point my development mail server to point to nowhere (smtp.gmail.com.dontSend) so no mail goes out and they all just stack up in undelivered. That way the rest of my development team does not get all the errors I generate and catch through email. More importantly, no test emails accidentally go out to real accounts.
I go into the CF-Administrator to look at and manage my undelivered mail when testing but if/when it gets huge I just delete them from the file system.