There is NO sender identity check on Internet email at all. Spammers do NOT need to hack into another person's account to send an email on his or her behalf. The address in the "From" field is as reliable as the back-of-envelop sender address in paper mail.
It is up to the senders to write whatever address they like to present themselves as. As a result, majority of junk email bears a faked sender address, which makes it hard to trace and block if at all possible.
You might have heard from your friend that he or she has received a junk email from you but you have never sent it. This is because spammers use your email address as the sender address because it is genuine and it is familiar to your friend and therefore easier to convince the recipient to open it. Also, if the recipient address is wrong, you will suffer from the bounce-back traffic, not the spammer themselves.
Unfortunately, there is little you can do to prevent spammers from stealing your identity. In many cases, you would not even know if your friends do not tell you and you have not received any bounce-back messages for email you have never sent.
Fortunately still, many email users these days would understand that the sender addresses of junk email are mostly faked. Some users simply delete junk email right away without looking into it, whether it is from someone they know or not. So the chance that your reputation is destroyed because spammers stealing your identity is low.