It tells you where it physically originated from, but doesn't tell you who sent it.
For example, if your friend lives in California but you receive a letter from New York, you may think the letter is a fake. The email domain name has a method of telling a receiving mail server where emails should be sent from, and if an email is received from somewhere else, it should be rejected.
Unfortunately, some mail servers (or their administrators) are unaware of this capability, and do not have this functionality turned on.
For example, you could put a California return address on a letter and mail it to a friend from anywhere, such as New York or China.
You could also put your friend's name and address for the return address and mail it from anywhere, even without your friend's knowledge.
San Francisco is also one of the most beautiful cities in the world with hilly streets providing gorgeous glimpses of the San Francisco Bay and its famous bridges.