[Exim] [Spoofed Mail addresses spamming mails]

Dear List

I have a problem in my exim server, spammers are telnetting to my server
using port 25 and making local delivery to unexisting email accounts ,
which in turn my exim server returns a error message to the sender and
most of those mails are bounced and comes back to my postmaster account.

I have smtp auth and open relays closed, as this is a local delivery
mail issue, how to curb this?

Thanks in advance

Vibhav