JANET, which routed email to Czechoslovakia, as the legend goes...
Back then mainly the Computer Science departments had email, so they'd have domain names beginning with a cs. in the ARPA scheme, but, since JANET did it backwards, you'd have to rearrange the domain name so it ended with a .cs for that network. If you did that and didn't reverse it back, the domain name would have a ccTLD of .cs, which is what Czechoslovakia used.
(The .cs ccTLD existed until 1995, years after Czechoslovakia ceased to. The .su ccTLD (Soviet Union) still exists.)
Back then mainly the Computer Science departments had email, so they'd have domain names beginning with a cs. in the ARPA scheme, but, since JANET did it backwards, you'd have to rearrange the domain name so it ended with a .cs for that network. If you did that and didn't reverse it back, the domain name would have a ccTLD of .cs, which is what Czechoslovakia used.
(The .cs ccTLD existed until 1995, years after Czechoslovakia ceased to. The .su ccTLD (Soviet Union) still exists.)