Just before the holidays, (sometime around December 17th-ish) Comcast changed their spam filtering process.
The bad news is that some of you may have experienced a larger than normal hard bounce rate to your recipients who use Comcast, and as a result, saw a lot of your recipients unsubscribed from your list. If you see tons and tons of Comcast bounces from your recent campaigns, contact our customer service team—they’ve got a retroactive kinda fix that can "un-bounce" erroneous unsubs, and get those people back on your list (unless your campaign was the one that caused the block in the first place).
The timing of all this, just before the holidays, wasn’t a very Comcastic thing to do to email marketers, but I guess those guys deal with a lot of spam. Who can blame them. Our customer service team was working all throughout the holidays to track affected IPs, re-route emails, and deal with the erroneous bounces. Whew, that was rough.
The good news from all this is that Comcast now has a feedback loop, and MailChimp is on it. That means from now on, whenever a Comcast recipient clicks his "report this as spam" button upon receiving your email, we’ll get a little notification, and we’ll instantly remove that person from your list to prevent future complaints. Now that’s comcastic! Sorry, I couldn’t resist.