failto attribute of <cfmail tag

The failto: (known as return-path) only allows one address as the property is setting the address of the originator of the e-mail, not for reporting purposes. If you need multiple recipients, set up an e-mail group; even though this is not the correct use, it could be used for that.

4.3.1. RETURN-PATH

This field is added by the final transport system that

delivers the message to its recipient. The field is intended

to contain definitive information about the address and route

back to the message’s originator.

Note: The “Reply-To” field is added by the originator and

serves to direct replies, whereas the “Return-Path”

field is used to identify a path back to the originator.

While the syntax indicates that a route specification is

optional, every attempt should be made to provide that information in this field.

Taken from:

http://www.w3.org/Protocols/rfc822/

Do you have correct mail settings in the CF Administrator?  Maybe CF is trying to use those credentials to send failure email.

Check your mail log and see why it is in the Undeliverable folder.  There might be something wrong with the email that prevented an attempt to send it.

BKBK is partially correct, I think.  My obervations are that if the recipient's mail server can't find the email address, you get a delivery failure notification and nothing goes to your undeliverable folder.  However, if the mail is rejected as spam, the opposite occurs.

Also, delivery failure notifications are not always instant.

I think failto will work if only the intended recipient's mail server sends back a mail delivery failure notification.