Apple's Mail app now uses a proxy to hide a user's IP Address when downloading images from emails.
However, it won't work if there's an active VPN connection. As it treats AdGuard as a VPN, it won't preload images automatically.
Apple explains this issue here.
AdGuard for Mac now uses macOS built-in socket filtering based on the network extensions API. This new and rather buggy mechanism replaced good old Kernel extensions. Over the last 1.5 years, we've reported more than 20(!) bugs to Apple regarding their new filtering method.
The network extensions API has a VPN-like configuration with a list of route-like entries.
On Big Sur, AdGuard used "split-tunnel" rules to avoid creating the "default route" rule because it causes problems on early Big Sur releases. These problems were solved in Monterey so nothing prevents us from using the "default route" rule.
On Monterey, iCloud Private Relay got introduced. Privacy features of Mail.app also use Private Relay servers.
As a result, AdGuard can't work together with iCloud Private Relay and Mail.app privacy features:
network.extension.monterey.force.split.tunnel restores the "Big Sur" behavior, but this option may break access to websites due to (3) and (4).
We keep searching for a solution to this issue. One of the options is implementing HTTP/3 filtering.
At this point, we recommend using a more traditional VPN service, such as AdGuard VPN, instead of the new Apple's privacy features.