How is thunderbird connected?

Question

System administration

Friends, the question is, you need to locally kill the entire Internet on your computer (it’s local, it’s the turn of hell, or you can’t mikrotiki)
The first thing that came to mind was to register dns 127.0.0.1, and add the necessary IPs through hosts. Added smtp, Yandex imap too, but the catch is that Thunderbird connects first somehow. You need to understand the IP address or where it connects so that these software can be used. Any ideas? Thank you very much in advance!)

Answer 1

the task is not to kill the network at all, here the HARDWARE probably would have guessed ....
maybe the default gateway is localhost or non-existent, and the route to the network is handled?

Answer 2

Erase the default gateway. Local will work, and everything outside of it will simply have nowhere to go. Or put the left gateway.