I wrote:

I get periodic reports from my service provider, listing all blocked junk
mail. There are at least 10 spams per week where the sending address is my
own address. How is this possible?

Even you can specify whatever e-mail address you want in your e-mail
client. So can a spammer. So far, mail servers do not verify that the
e-mail address you specify as your own actually matches to the e-mail
address of the account through which you send out your e-mails.