How to disable a domain

Sometimes you might want to make it clear that a domain isn’t valid for email.
Perhaps it’s a domain or subdomain that’s just used for infrastructure, perhaps it’s a brand-specific domain you’re only using for a website. Or perhaps you’re a target for phishing and you’ve acquired some lookalike domains, either pre-emptively or after enforcement action against a phisher, and you want to make clear that the domain isn’t legitimate for email.
There are several things to check before disabling email.
1. Are you receiving email at the domain? Is anyone else?
Check the MX records for the domain, using “host -t mx example.com” from a unix commandline, or using an online DNS tool such as xnnd.com.
If they’re pointing at a mailserver you control, check to see where that mail goes. Has anything been sent there recently?
If they’re pointing at a mailserver that isn’t yours, try and find out why.
If there are no MX records, but there is an A record for the domain then mail will be delivered there instead. Check whether that machine receives email for the domain and, if so, what it does with it.
Try sending mail to postmaster@ the domain, for instance postmaster@example.com. If you don’t get a bounce within a few minutes then that mail may be being delivered somewhere.
2. Are you sending email from the domain? Is anyone else?
You’re more likely to know whether you’re sending mail using the domain, but there’s a special case that many people forget. If there’s a server that has as it’s hostname the domain you’re trying to shut down then any system software running no that server – monitoring software, security alerts, output from cron and so on – is probably using that hostname to send mail. If so, fix that before you go any further.
3. Will you need mail sent to that domain for retrieving passwords?
If there are any services that might have been set up using an email address at the domain then you might need a working email address there to retrieve lost passwords. Having to set email back up for the domain in the future to recover a password is time consuming and annoying.
The domain registration for the domain itself is a common case, but if there’s any dns or web hosting being used for the domain, check the contact information being used there.
4. How will people contact you about the domain?
Even if you’re not using the domain for email it’s quite possible that someone may need to contact you about the domain, and odds are good they’ll want to use email. Make sure that the domain registration includes valid contact information that identifies you as the owner and allows people to contact you easily.
If you’re hosting web content using the domain, make sure there’s some way to contact you listed there. If you’re not, consider putting a minimal webpage there explaining the ownership, with a link to your main corporate website.
5. Disabling email
The easiest way to disable email for a domain is to add three DNS records for the domain. In bind format, they look like:

example.com.                  86400 IN MX 0 .
example.com.                  86400 IN TXT "v=spf1 -all"
_adsp._domainkey.example.com. 86400 IN TXT "dkim=discardable"

The first record says that the domain accepts no email. This is a standard part of SMTP, discussed in more detail in this internet draft. One corollary of this is that the domain will not be able to accept bounced email, so most mail filters will consider any email claiming to be from the domain as illegitimate and will reject or discard them.
The second record uses SPF to say that there are no servers that will legitimately send mail for the domain. This is not as widely supported as the first record, but adds an additional level of protection, and will make it clearer to people not familiar with the “MX 0 .” idiom that you’ve intentionally disabled email.
The third record uses ADSP to say that any mail sent from the domain that’s not cryptographically signed should be discarded. This is also not widely supported, but again adds another layer of protection.
In some rare cases you might want to reject email sent to the domain with a specific message, rather than with a generic rejection. The best way to do that is to replace the first record (“MX 0 .”) with an MX record pointing at a mailserver that’s configured to reject all recipients at the domain with a customized 5xx rejection message.