I think email marketing is a dead end nowadays. That's what was done here, when you changed all members options and forced receipt of admin email. That is vastly different from opt-in email marketing, which has great success.
Have people stopped using email? No, it's widely used.
Have people stopped opening things in their email that they otherwise don't expect? Absolutely, due to the global trend of electronic fraud, most people globally are becoming highly cautious with email they do not normally expect... and your experiment did exactly that, being you overrode member settings to not receive email, then they received it, and an announcement on your forum isn't going to do a thing due to the majority of your members don't use your forum now, as people move on, and those that do, who actually took any notice of the announcement and remembered it as important when receiving the email that they are not normally receiving from your site!
I would say your experiment methods were wrong from the start.
If you're going to perform an experiment with email marketing as you have, then you need to do it with an opt in list of people who actually say they want to receive your email, thus they expect it in their inbox and not delete it as spam or such. You're better off to send out 30 emails to users who want them, then measure that click through rate, than 20k plus to every member of your site, most of which aren't even active any longer, and will delete it or as you said, no longer use that email address / it went to spam directly.
If I did that on my site, even with an announcement, I would expect little positive results, as the majority are overtly cautious of emails already from discussions on such topics due to online fraud / already being defrauded at some point / identity theft at some point.
I think a better experiment would be to advertise across your site a new monthly newsletter, giving people time to change their settings or using an external opt-in / opt-out email program that can track as accurately, then knowing how many are being sent, use that figure to measure success.
You increase opt-in members to such monthly newsletters by advertising special promotions only available to subscribed readers / free upgrades, etc etc... that way active members subscribe because they want something for nothing, even if it is only one free upgrade to the first person who does x.
