Settings in/on your email client such as thunderbird
To elaborate, if you use an email client like Thunderbird (as just one example), someone could write a script, or malware that you unknowingly download. IT then accesses your data and sends emails as if they were you.
SOMETHING I forgot to mention to the OP. It is possible that a friend could receive an email that appears to be from you, but in actuality could be from someone else's computer that you've sent mail to.
When sending to your recipients, that data is embedded in the body of the email. Malware can that that info from ANY other computer that mail was sent to, and if that computer is infected, could send mail that appears to come from someone completely different from what is shown in the header of the email.
This subject is simple, but the actual events that take place from point A to point B can be variable.