Hello Mrs Dog, What a shame if you have sent seeds to people who don't acknowledge receipt or thank you. I too have sent seeds to people on several occasions who did likewise. You just have to accept it really doesn't matter. I don't know what's wrong with those people but don't let it change you. Carry on in your own kind way and let them go their own miserable way.
Or the other thing is, something quite dramatic could have happened in their lives - sickness or a bereavement - and they didn't have time to deal with it. Give them the benefit of the doubt and carry on as you are.

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From each according to his ability, to each according to his needs.