How to send an entire domain to /dev/null in Postfix

At Exoweb, our software developers use bogus email addresses of the form *@example.com (where I mean “example.com” literally, not as an example) to test their software’s ability to send email. Since I don’t want our Postfix server to attempt to deliver these messages out on the Internet, I need Postfix to handle these messages and blackhole them (make them disappear, sent to /dev/null). So what follows are instructions on how to blackhole an entire domain in Postfix.

First, we add a virtual_alias_maps entry to /etc/postfix/main.cf so that we can specify example.com as one of our virtual domains:

virtual_alias_maps = hash:/etc/postfix/virtual_alias

Inside /etc/postfix/virtual_alias, add a catchall address:

@example.com      blackhole@localhost

We have to use blackhole@localhost here and not /dev/null/ because virtual_alias_maps cannot run commands—it can only forward to real addresses. So we put an entry inside /etc/aliases to handle the blackhole:

blackhole:      /dev/null

This assumes that one of your mydestination domains in main.cf is localhost so that Postfix will actually consult the aliases file.

In order to make these changes take affect, you have to rebuild the aliases database, build the virtual_alias database, and reload your Postfix configuration. Respectively:

# newaliases
# postmap /etc/postfix/virtual_alias
# postfix reload

Now, any emails you send to blackhole@localhost will disappear, and so will any emails addressed to anyone@example.com (provided they are relayed through your Postfix server).

Footnote: The top-level and second-level domain names that are reserved for testing can be found in RFC 2606.