The redirect feature allows aliases to be set up for retired accounts. Those aliases bounce with an indication of the new forwarding address. A couple of lines from such an aliases(5) file might look like this:

george:   george@new.site.edu.REDIRECT
william:  wc@creative.net.REDIRECT

The redirect feature causes mail addressed to george, for example, to be bounced with a message such as this:

551 5.7.1 User not local; please try <george@new.site.edu>

Note that the message is bounced and not forwarded. No notification is sent to the recipient's new address.

The form of the redirect feature is:

FEATURE(`redirect')

The actual bounce is caused by calling the error delivery agent with an RHS such as this:

$#error $@ 5.1.1 $: "551 User not local; please try " <$1@$2>

The 5.1.1 is a DSN error code (see RFC1893), and the 551 is an SMTP code (see RFC821).

If your site's policy is to notify and forward, you can use an entry such as this in your aliases database:

george:   george@new.site.edu.REDIRECT, george@new.site.edu

Here, the sender will receiver notification of the new address, and the recipient will receive the original messages.

A problem can arise when spam messages are sent to a REDIRECT address. Because some spam is sent with a fictitious envelope sender, the bounce caused by the REDIRECT will itself bounce too. This creates what is called a double bounce (a bounce notification that bounces). Double bounces are delivered to the address defined by the DoubleBounceAddress option (see DoubleBounceAddress). If spam bounces of REDIRECT addresses start to annoy you, consider redefining the DoubleBounceAddress option to deliver double bounce notification to a less offensive address, such as an address aliased to /dev/null. But be aware that this will cause all double bounces to be sent to that address, not just spam double bounces.