An email message is very similar to a physical letter that you would send in the mail. There is an envelope, with To/From information, and there is the actual letter on the inside, with its own To/From information. The envelope to/from information is the real information that is used for message delivery, for both email servers and post offices.
When an envelope comes into a post office, they inspect the To address on the envelope, and send it to the correct destination. The post office workers have no knowledge of the letter inside the envelope. The letter inside could have completely different To/From information than the envelope says. The Envelope could say the message is to Joe, but the letter inside may say it's for Mary. Or, the envelope may say it's to Joe, but the letter inside says the message is for Joe and Mary.
The same is true for emails. The envelope To/From is the most important information when delivering a message. If the envelope says the message is to firstname.lastname@example.org and email@example.com, then the message is sent to both, regardless of what the letter inside (the message header) says the message is "To:". So the message may appear to be for some random person, but actually come to you.
If no Envelope sender is specified, then the Envelope sender is the same as the From address: