Customize / edit SMTP delivery status notifications
Generic delivery responses are useful to mail admins, but rarely helpful to end users (and sometimes outright rude).
For example, "5.7.0 Your server IP address is in the SORBS DNSBL database, bye." A more useful response would provide the recipient with instructions on how to resolve the issue.
I'd like to have a GUI listing of all the possible server responses with the option to change them to better inform the recipient. Ideally, the interface would provide a "revert to default" button, or simply show the default along side the modified text.