Vivio Technologies has a couple clients who use Railo to send mail to their mailing lists. Some of the email addresses on these lists become inactive, and no mail is able to get sent to them. When this occurs, these emails get "stuck" in the mail queue and can never be delivered (at least this is what happens to the best of our knowledge).
It would be great if the Railo Administrator provided a way to customize how emails that cannot be sent are handled. For example, after three attempts to deliver, the emails are sent to an "undeliverable" folder (which is what ACF does) - or something similar.