It may be possible that the "one recipient's" email address is either incorrect, has been changed, or purposely blocking you from sending them emails. The key now is finding out which one it is.
I suggest that you start by first verifying that the Mail app on the phone is not the culprit. To do so, try using a Mac or PC, or another iOS device to send out emails. Does this particular recipient still get flagged? If so, then you know that it's their email account. If not, we can try some additional troubleshooting on your phone.
In the mean time, you may find the following article helpful: