"Soft Bounce: Challenge-Response." This was a new kind of soft bounce to me, I didn't see it in the "Tell me more"/"Definitions" page. Any info on what it means or best practices to deal with? Thanks,
Hi Jeremy, challenge-response is a form of spam filtering in which generally the spam filter replies to the message with a verification email, which requires a response before the original message can go through.
There's a possibility that you've received an email reply to your From: or Reply-to: email address. This will have to be responded to before the campaign can be delivered to this particular subscriber.
There isn't much you can do on your end, except to use a valid From/Reply-to email address and keep an eye out for verification email. In my experience, not many folks use challenge-response spam filters these days thankfully, as modern spam filtering is fairly sophisticated and it's always an annoyance for legitimate senders.
Update (09/08/13): We actually bounce these challenge-response emails, meaning that they don't get sent to the reply-to address. While we're looking at addressing this, it hasn't yet posed a significant enough issue to be prioritized - it just doesn't happen enough times to impact campaigns significantly. However, if you feel this isn't the case, please get in touch with campaign details and we'll certainly look into this further.
Cheers, jeremyhr - I hope that answers your questions there :)