Hijacked by Spammers

My zoho account seems to have been hijacked by spammers... I’m repeatedly getting Delivery Status Notifications of undeliverable mail. The undeliverable e-mail has my zoho address as the sender, but they’re nothing that I sent; they’re to people I don’t know, and they’re for a weight-loss product.

How do I stop these spammers from using my account?!!!

David