:-)
SMTP_MAX_RCPTS = 1 (http://www.mailinglistarchive.com/mailman-users@python.org/msg08741.html -- thanks Mark!) helped. Still I'mnot sure why :-(
It helps because of a bug in the underlying Python smtplib. See <http://bugs.python.org/issue5713>
Your real problem is you have addresses on your list that Postfix thinks are undeliverable. The bug report linked above describes the scenario.
After 20 invalid local recipients in one SMTP transaction, Postfix disconnects with a 421 Too many errors reply, but the Python smtplib module doesn't expect a disconnect after a 421 and gets out of sync with Postfix.
Setting SMTP_MAX_RCPTS to any number less than 20 will avoid this problem.
THANKS A LOT!!! Now it works and I do know why! Wonderful!
Best tregards, Marek Kozłowski