We send out a weekly mailing to several thousand subscribers using an old version of Lyris ListManager, and have done so for years without incident. Every month or so starting about a year ago, two or three subscribers from one specific ISP complain that they're receiving the message up to a dozen times each day. Our logs show the messages as being successfully sent on their first try, the "duplicate" messages all have the same time stamp, and we've determined the responsive subscribers were probably suffering from their first corrupted popstate.dat file. The experience is apparently so frustrating that most of them unsubscribe, which isn't acceptable because we like our customers. This effects zero users at any other email provider.
I'd be content with blaming this specific provider, but it's just too uncanny that it's the same weekly mailing and only one email provider, and these subscribers aren't suspicious enough of their own email provider to suggest these kinds of corruptions are common.
Is it possible something in our message is causing these corruptions? Could their email service be uniquely configured and consistently garble the message ID being saved to popstate.dat, causing a subscriber's popstate.dat file to store an incorrect message ID and continuously allow their email client of choice to download our mailing?
