How are Your SMTP Timeouts Configured?

Asprin asks: "One of the employees at work had a major headache because a very important email was undeliverable for more than 24 hours. Sure, he got an warning from our server about it, but only after an entire day had passed, and the email was no longer timely. Therefore, I shorted the message handling timeouts to send 'delivery delayed' warnings after 30 minutes and to cancel the message as undeliverable after four hours. Now, I don't expect any of the other mail administrators here to bless these timeouts because they're way too short. HOWEVER, the truth is that my users rely on email to be as reliable as telephone messages, and if it can't be delivered immediately, it is better to reject it outright and alert the user so that other communication channels can be exploited such as fax or Fed-Ex. Is anyone else doing this? Are there any non-obvious ramifications lurking? Pros? Cons? Comments? Should we all reduce these timeouts on our SMTP servers?"