I was let go almost 2 years ago and my old employer is still sending emails, as though they were from me, to my old customers. Is that legal?
It’s not, and never was, your email address
And, incidentally, the emails sent and received by it are not your emails. They all belong to your employer.
For a time, they allowed you to use it. Now, someone else is using it. No laws are being broken.
However, if they are using it in such away to create the impression that you still work there or in a way that damages your reputation then we are heading into fraud and defamation territory respectively. The first is none of your business, that’s between your ex-employer and whoever they are sending emails to. The second you can bring suit over.