It seems like this has been requested many times. See:
https://github.com/k9mail/k-9/issues/760
In fact, there looks to be a pull request that may implement this functionality, though it hasn't been touched in a while:
https://github.com/k9mail/k-9/pull/2697
There looks to be requests for review, so perhaps you could test out that PR and add your results to the comments?
Note that I am not a dev for K-9, nor do I know the devs' sentiments on this idea, nor do I know the quality of this PR. Just giving the results of a quick search. Hopefully this points you in the right direction.
--Sean