"Software caused connection abort" and "Socket closed" are typically caused by changing connectivity, which includes restarting the Android VPN service, which is done to apply a new rule set (switching from metered to unmetered, turning the screen on/off). This cannot be fixed or improved. NetGuard as no-root app can only use the standard Android APIs and these are limited.
"Connection reset by peer" is typically done by the remote server and not related to NetGuard.
Related FairEmail FAQ:
"Couldn't connect to host" is likely caused by blocking FairEmail with NetGuard.