Hello,
it seems strange that Acymailing would trigger the 404 Shield block, such feature kicks in when a visitor tries to reach some WordPress pages on your Joomla website (ie wp-admin, wp-login.php etc etc).
I suspect you have a proxy in front of your server, so you're sharing the IP address with other visitors, triggering the block and causing all the people to get blocked.
Please run this quick test: visit the URL https://whatismyipaddress.com/ and write down the IP you see on screen. Then get inside Web Application Firewall, Administrator Exclusive Allow IP List and click on New. In the new page you will find the IP that Admin Tools is currently detecting.
Does it match with the previous one?
If not, it means that you have a proxy in front of you, but it's an easy fix. Get inside the Configure WAF page and on the first tab set the feature Enable IP Workarounds to Yes.
That should fix your issue.
Davide Tampellini
Developer and Support Staff
🇮🇹Italian: native 🇬🇧English: good • 🕐 My time zone is Europe / Rome (UTC +1)
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!