To combat phishing, Swisscom sends its e-bills with an electronic signature (e-certificate). The signature is used like a digital signature so it shows that the e-mail has actually been sent by Swisscom.
How you can tell if the e-mail has an electronic signature:
The way the electronic signature (e-certificate) is checked and displayed depends on the operating system you use.
For more detailed information please consult your e-mail software manual or read the information provided by the federal government