I do not use K9mail and would like to know what this means:

Looks like the email app you’re using is based on (quite an old version of) K-9 Mail’s source code. Please contact them for support.

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This application is called “Email”. It is based on k9. It is no more in any store, I think.

More precisely, it is “Email mail box fast mail”, version 1.15.278 from BACHA soft. It is available on Apkpure.