This is mostly solved - 6.3.10 and newer have a "softbounce" option that can be set to keep mail on the POP3 server even if the SMTP server refuses such mail.
However, I still think that such illegit mail should not be allowed to clog up the inbox and should be deleted. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org