The latter. What we're talking about here is a TLS negotiation. So, when you connect to a TLS server, in this case to deliver email to Gmail over SMTP - you te

The latter. What we're talking about here is a TLS negotiation. So, when you connect to a TLS server, in this case to deliver email to Gmail over SMTP - you tell it what ciphers you know [the RFCs list "Mandatory to implement" ciphers, but the IETF do not have an enforcement arm, so you could choose to list only ciphers you made up here for whatever that's worth] - and then the server picks one from your list. Google's change here is when a client calls Gmail and professes that the least awful cipher it knows is 3DES, that's too bad, connection failed. So yes, this only affects email which was previously using this awful cipher and now just can't be delivered instead.