WeatherCat Version 4.0.0 early dev release #3 is now available for testing. See https://athena.trixology.com/index.php?topic=3677.0 for more details and download.
I have just updated my Mac Mini m2 to macOS Sequoia and WC is running fine (apart from the SFTP problem mentioned elsewhere)
Sorted - it was an Xcode problemClick on the Xcode app - accept and enter password - then let it update it's stuff and then restart WC - emails working fine again.