An undeliverable email is supposed to be canceled (with notification to the sender) after seven days. Sounds like the sender's ISP has a non-standard config on their SMTP server.

__________________
[Dr. Horrible]___________________________[Penny]
Listen close to everybody's heart________And you believe there's good in everybody's heart
And hear that breaking sound_____________Keep it safe and sound
Hopes and dreams are shattering apart____With hope you can do your part
And crashing to the ground_______________To turn a life around