If you catch yourself saying that something you ought to do isn’t your job, you’re probably not doing your job properly.
You weren’t hired to be a mindless machine (if you were, you should probably quit). You were hired as a problem solver. If a problem is not strictly in your area of expertise, but you can solve it, it is your job to do so.
If you start solving problems you’re not supposed to solve (unless it hurts your ability to solve problems you should solve), you’ll become more valuable asset of your company, and your company will become better at solving problems — which benefits everyone, especially you.
As a rule of thumb, you should consider everything even remotely related to your company as your job.