Well you are of course.
Then there are spammers, whom we try to control with SPF records in the DNS to limit where your email can come from to the servers you use.
Then there are misguided websites which try to use your email address to send mail when someone fills in a form claiming to be you. You didn’t send that mail, the website did, so the 4 players in the message should be set appropriately; they are
From — who he message purports to be from — you in this case
Sender — the website administrator
Reply address — again probably you
Error Reports — Probably should go to the website admin
Websites will usually fix this once they understand the problem.
Finally there are people doing stuff for you, for instance payment portal providers, or webservice consolidation sites such as house or employment search services. Most of the above applies, with the added wrinkle of you as the named responsable (EU French term for the person accountable) for the service.
The portal will correctly show you as the From address, but should not be using your email as the sending address that’s just simply wrong, if you do agree to it then the portal should provide a portal SPF record that can be included by your own SPF record, how otherwise are you supposed to maintain your email security with random contractors thinking they can send as though they are you vs someone providing a service to you?
One does not use another company’s headed notepaper after all. That’s fraud. Why then should you use another company’s email address, without making the ‘on behalf of‘ relationship clear?