How do you delete email histories of addresses to which you only sent 1 email and have no need for a list of 1-timers?

Unfortunately, you've run into one of the "helpful" features of Apples that isn't. As in, you can't remove those e-mail addresses. All you can do is hope that after a certain time (which is unknown and unknowable), those e-mail addresses drop to the bottom of that e-mail pop-up.

This "feature" has been a bane of many here at this forum. All we can hope is that eventually Apple makes an update that will finally allow us to remove those addresses.

Marilyn