How do challenge response systems affect senders and third parties?
Summary
What email marketers say5Marketer opinions
Email marketer from Mailjet shares that if you are using a challenge-response system you are significantly hurting your deliverability with all major mailbox providers. It will also mean you are more likely to get false positives and your real emails missed.
Email marketer from StackExchange explains that challenge-response systems negatively impact deliverability. Legitimate recipients may not complete the challenge, leading to lost emails. Additionally, many email providers automatically classify emails from such systems as spam.
Email marketer from Reddit explains that challenge response systems affect accessibility for people with disabilities, the captchas are difficult to read.
Email marketer from Reddit explains that in a high volume situation challenge/response systems will overload the server as it requires processing for every email that is blocked.
Email marketer from Quora shares that challenge-response systems create a poor user experience. Requiring senders to complete a challenge adds friction and can deter legitimate communication. Many users find these challenges annoying and may simply give up on sending the email.
What the experts say4Expert opinions
Expert from Word to the Wise explains that challenge response systems can hurt SEO because search engine crawlers will not fill out the challenge and therefore cannot index the site. This can result in lowered search engine rankings.
Email marketer from Email Geeks explains that using challenge response systems can inconvenience innocent third parties if the original sender used a forged address.
Expert from Spam Resource shares that Challenge/Response filtering sucks and that they inconvenience third parties.
Expert from Email Geeks shares that they approve challenge responses for mail they didn't send, viewing it as the sender outsourcing their spam filtering.
What the documentation says3Technical articles
Documentation from Microsoft Learn explains that challenge/response systems often generate backscatter. Backscatter occurs when a spammer spoofs the sender address for a message, and the challenge/response system sends a challenge message to the innocent, spoofed sender address, flooding them with unwanted email.
Documentation from RFC explains the challenge response system does not comply with Sender Policy Framework (SPF). SPF helps prevent sender address forgery and relies on the sending server's IP address. Challenge-response systems often forward mail from a different IP address, invalidating the SPF check and potentially causing deliverability problems.
Documentation from Spamhaus explains that challenge/response is a poor filtering technique because it bounces spam back to forged sender addresses (backscatter).