One correspondence was from Ellen, deputies at admin.spamcop.net:
Ah, OK this is challenge/response -- that is what I was not clear about and
now understand. If you continue to send C/R emails to the spamtraps then
there is always the possibility of the IPs listing/relisting.
Unfortunately our system is also automatic, and I can only assume that spammers forge these spamtrap addresses so that our system tries to verify them then gets blacklisted. I highly doubt they will whitelist our IP, since I bought up that issue in the E-mail to Ellen and she did nothing about it, but we can keep trying.
There are people reporting the welcome message as spam? That is sad.
We are reluctant to buy a new different IP block to send the verification messages, since many mail servers might reject the mails for a different reason: the domain "0spam.com" (in the From headers) would not match MX records, and I know that some servers use this matching to block spam.