I don't believe this to be a problem specific to Campaign Monitor, but thought the community may have some experience with it, so I'm throwing it out there.
In the last 2-3 weeks we've been getting odd replies to our subscription confirmation and welcome emails being sent through Campaign Monitor. A reply will be sent to us with just "Not bad" or "Interested?" in the email body. Nothing else. It doesn't seem to be specific to any particular email provider as we've received these odd replies from Yahoo and AOL email addresses.
My guess is that these are all spammers signing up for our list, though I can't figure out what their end game is. Has anyone experienced this?