There is no e-mail contact available. You can call the SBB Travel Center at +41 51 225 78 00. The announcement might be in German, just stay in line until someone answers.
Alternatively, you can book your tickets with Trenitalia, from Spiez to Milan (Grindelwald cannot be chosen as a starting point). Once you are in Switzerland, you can buy the connecting tickets from Grindelwald to Spiez.