I get a lot of email. So do you, probably. Much of the email I get is from people I don't know well, and I'm happy to receive it. People I don't know well usually don't know me well, either. Often, we've never corresponded before. That is one of the wonderful things that I love most about the work I do.
Once in a while, one of these exchanges passes through a kind of spam protection device called a "Challenge/Response" system (CRS). And at that point the exchange becomes burdensome to me. It works like this. Someone with whom I have never corresponded before (or not recently, anyway) sends me a message. I reply. My reply goes through that person's CRS. The CRS notices that my email address is not in the person's approved correspondents list (sometimes called a whitelist or greenlist). The CRS then sends me a message asking that I click a link to fill out a form that will then be processed in some way by the CRS, and then my message will go through. This is already above my pain threshold, but hang on, it gets worse.
The form I have to fill out usually contains one of those funny little graphics of distorted letters and numbers (called a "Captcha"). I then have to figure out what it says and type that into the form. The captcha protects the form from being hacked by non-humans, ensuring that only humans will get through the CRS. And like you, I sometimes get it wrong the first time.
All of this rigmarole is designed to relieve my correspondent of the annoyances of spam, a goal with which I certainly sympathize. But the CRS scheme accomplishes this by exporting annoyance onto the CRS user's correspondents. And I do not sympathize with that. It works OK for people who correspond only with others who receive email almost exclusively from those they know well, but otherwise it's burdensome to their correspondents — especially to people like me.
That's why I rarely respond to Challenge/Response systems. If you use a CRS, and you're expecting a message from me that hasn't arrived, the probable explanation is that I didn't respond to your CRS. Many of these systems have portals for you to examine blocked messages. I suggest that you look there.
Even better, if you use a CRS, and you send a message to someone with whom you don't usually correspond, please take the time to greenlist or whitelist the person's email address(es) before you click Send.
Even better than that, dump your Challenge/Response system.
If you want to read more about this issue and its impact on people like me, check out this article by Christopher Breen, "Earthlink and the devil's spam filter" or a blog entry from James Fallows of the Atlantic, "If you write me from EarthLink, here's why I won't write back."