(UBE) by the receiving system, possibly leading to blacklisting of the mail server or even listing on a DNSBL. In addition, if the forged sender decided to validate the challenge, the C/R user would receive the spam anyway and the forged sender address would be whitelisted. Though definitely an undesirable side-effect, this issue would be non-existent if people, whose email address was used as a forged address in spam, happen to run a C/R system themselves. In this case, one of the C/R users must implement some form of return address signing (such as Bounce Address Tag Validation) to ensure that the challenge goes through. Also, if systems like SPF and DKIM became common, forged sender addresses would be recognized by these systems before reaching a C/R system. In some cases, C/R systems can be tricked into becoming spam relays. To be useful, some part of the message under challenge is generally included in the challenge message. A spammer, knowing that he's sending to a C/R using system, could design his message so that his "spam payload" is in the part of the message that the challenge message includes. In this case, the forged sender is the actual recipient of the spam, and the C/R system unwittingly is the relay. Social issues Disseminating an ordinary email address that is protected by a C/R system results in challenges to those who send mail to that address. Some C/R critics consider it rude to give people your email address, then require them (unless previously whitelisted, which might not always be possible) to answer the challenge before they can send you mail. Advocates of C/R systems argue that the benefits by far outweigh the 'burden' of an incidental challenge, and that there will probably never be a final solution against spam without laying some kind of burden on the e-mail sender. They reason that the more widespread the use of C/R systems is, the more understood, accepted and appreciated they are. In an analogy with snail mail, the sender is prepared to pay for the stamp, in an analogy with phone calls, the caller is prepared to pay for the outgoing call. Interaction with mailing lists or other automated mailers Some C/R systems interact badly with mailing list software. If a person subscribed to a mailing list begins to use C/R software, posters to the mailing list may be confronted by challenge messages.