So I picked this topic up with Mandrill Support and with their permission I’ll post Jonathan’s relevant responses here in their entirety for your reference:
Jonathan (Mandrill Support)
May 7, 3:15 PM
Hey Dean,
Thanks for reaching out and providing those details. You’re absolutely right about AOL either deferring or outright bouncing some of your emails, which is also being reported by other users. While there isn’t too much information available to us about the source of the issue, I can confirm that this isn’t due to any issue on Mandrill’s end, and those bounces aren’t due to any blacklisting. Instead, it appears to be a more endemic issue on the receiving end that’s affecting emails sent to AOL from multiple platforms.
It’s also worth noting that the provided blacklist fortunately isn’t one we’d view as particularly reputable, and it’s unlikely to be used by major ISPs like AOL. When it comes to blacklists, we’d generally advise focusing on the largest, most reputable blacklist services—namely SpamCop—unless you’re specifically seeing emails blocked because of the listing.
We’re also actively trying to investigate this particular issue with AOL, but I unfortunately wouldn’t be able to provide a timeframe for when things might improve. However, definitely feel free to let us know if there are any questions, and we’ll be glad to help.
After requesting permission to post his response:
Jonathan (Mandrill Support)
May 7, 4:04 PM
Hey Dean,
No problem. You’re also welcome to convey this information to other users, but it may be worthwhile to include some caveats, particularly since this type of rejection is unfortunately vague and may cause confusion if the same diagnostic code is provided in the future for an unrelated issue.
However, I think it’s at least fine to mention in the forum that other users are reporting the issue, and that we’re confident it’s not an issue on Mandrill’s end or due to any blacklisting, possibly while referencing that the post is specifically in regards to bounces and deferrals that are occurring this week with the following SMTP replies (as AOL has other diagnostic codes, too):
521 5.2.1 : AOL will not accept delivery of this message.
421 4.2.1 "Service unavailable. Please try again later."
That’s should hopefully help avoid any confusion that might be caused by someone stumbling across the issue in the future and simply seeing my last message provided there.
I also wanted to add that Spamhaus is a particularly reputable and influential blacklist, as I noticed I forgot to mention them, previously. This article from MailChimp also does a nice job of explaining the differences between different blacklists, while listing a few other important ones:
I hope that information helps.