As widely reported, MAAWG, the Messaging Anti-Abuse Working Group put out a statement a couple of weeks ago basically condemning email append. Find a link to the statement here, or click here to read Laura Atkins' coverage of the announcement.
MAAWG is a broad ISP, email and network ecosystem-focused organization dedicated to stopping abuse. As email abuse (spam) is a big challenge, email spam continues to be one of the things that MAAWG members work together collaboratively to try to mitigate and otherwise address. There are lots of heavy hitters in the email space represented in MAAWG's public membership roster: Yahoo, Microsoft, AT&T, Cloudmark, Comcast, Time Warner, Spamhaus, and many others. These are the folks who decide whether or not your mail gets through to the inbox, is sidetracked to the bulk folder, or blocked outright. And they're standing up and saying, flat out, email append is a bad idea.
As a followup, industry agitator (and I mean that term with respect) Ken Magill shared his thoughts: That the message from MAAWG was painted with too broad a brush. That not all email append is bad news. A statement I agree with ... sort of. And MAAWG probably has no issue with it, either. If it's all truly opt-in. Confirmed opt-in before any sort of data transfer happens.
But, as I pointed out when Ken interviewed me for his second followup, the colorfully titled "That Sound You Hear is my Head Coming Out of my Ass," nobody ever does email append that way! I'm aware of exactly one single retailer bothering to go through the trouble of doing opt-in email append. Why so few? I think that it's because the email append vendors don't want to do it this way; an opt-in process means fewer matches and fewer new subscribers, thus, a smaller amount of revenue for the append provider.
I've actually seen these guys try to tell people that opt-in is overkill, but the truth of the matter is, when somebody goes off and does email append, against my advice, they end up with deliverability problems down the line. Use email append, spam complaints rise, and you get blocked at Yahoo. It's pretty much that simple. If you've done email append and not have that problem arise, it's probably because you did it on such a small scale that you were able to stay below the radar. Congratulations! You didn't get caught when you broke the rules! That's hardly a scalable or wise business practice, however.
But don't take my word for it. Microsoft's Terry Zink just posted a very easy-to-follow explanation of what actually goes wrong when you do an email append.