The FairEmail client has the option to use only "validated" connections. The only explanation given in-app is that a VPN might prevent synchronising from working if this option is enabled, but what does this actually mean? Is this about not using the very weak 2G/3G encryption or open/WEP WiFi?
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In the source code I see this sets a preference which is later read and enables the checking of NET_CAPABILITY_VALIDATED. This capability is documented as "Indicates that connectivity on this network was successfully validated." which tells you absolutely nothing. However, an example is given for "a network with NET_CAPABILITY_INTERNET, it means that Internet connectivity was successfully detected." Digging into when that capability is set: "Indicates that this network should be able to reach the internet."
So if it expects that a connection should reach the internet, some part of the OS will check it, and then apps can check that the check was successful, and only then they will try themselves to talk to the internet. That's what this switch does.
Presumably this will prevent custom setups from working, such as when you change routes after making a WiFi hotspot (without mobile data enabled) where it wouldn't expect to be able to reach the internet regardless of whether it can. Or if you, indeed, have a VPN where the email server is reachable but the internet in general is not.
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