The fact that the report mentions a daemon (maild) means that it may not be possible for an end user on a non-jailbroken device to stop activity. (For a non-Apple app, simply force-quitting by swiping up in the switcher would go 99% of the way.)
I'd guess that removing mail accounts from the device would do the job, though.
It's possible to delete the Mail app (which I had already done on my phone). I'd hope that would kill related daemons. I'm jailbroken as well so I'll check if the daemon is running later tonight.
I'd guess that removing mail accounts from the device would do the job, though.