WIP: Verify recipient validity at RCPT command in LMTP runner

fox.cpprequested to merge
foxcpp/mailman:rcpt-verify into master

Implementation for #14 (closed). Not based on existing WIP MR.

P.S. as a person barely knowing Python, I failed to run tests locally due to some obscure error. I hope CI can help me figure if it is working :) Hence. WIP: .

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