Very high CPU usage in Mail.app

I’m seeing very high (100%) CPU usage in Mail.app.

The behaviour is a bit random. Typically it will work properly several hours and suddenly I will notice my MacBook gets hot. Most of the time it happens at some point during the night and I find it hot in the morning. It seems that Mail.app has trouble connecting and gets stuck in a loop somewhere. I have to force quit it.

I think it started with AdGuard 2.6. It started on Big Sur, and still happens in Monterey.

This seems similar to #928.

I have no antivirus, firewall or other network related software installed. I’m running a fresh 12.0.1 install on an M1 MB Air.

I sent a bug report. Ticket number is 548998.

Below is a screenshot of Mail.app with an error message saying the connection expired. It _think_ iCloud and Yahoo (top and bottom) are affected but not Outlook/Exchange.