Verify recipient validity at RCPT command in LMTP runner (2nd try)
Verify recipient validity at RCPT command in LMTP runner (reviewed merge request
c2ddff05a4f405fa46fb792cc69912829c1cbf83, rejected 2 years ago)
* returning '250 ok' to correctly accept valid recipients
* reorganized code a bit to better match existing recipient verification code in handle_DATA()
* fixed unit tests (verified with `tox -e py39-nocov` on Debian/bullseye)
See !671 for the original merge request by @foxcpp. Also see !126.
Fixes #14