Why do I keep getting failed Email Delivery Notices?

Hi

I've had a mac.com email address for many many years and not had any problems until just recently.

I keep getting emails from postmaster@mac.com saying that my email falied to be delivered. These are emails that I have not sent and there are loads of them. This morning I got about 80 of them. It started a few months ago so I changed my email account password to a much stronger one. It then stopped happening but started again last week!

Does anyone know what what is going on and how I can stop this properly?

Thanks