What is domain literal addressing?
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Domain literal addressing is when you put an IP address in square brackets instead of a domain name in the recipient part of an email address. So captain@[192.0.2.44] instead of captain@harborpost.net.
It was designed decades ago for debugging mail servers and testing network routes. The idea was you could skip DNS lookups entirely and send directly to a specific server IP. Useful for network admins troubleshooting mail flow, less useful for literally anyone else.
Why you'll never need this: modern mailbox providers reject domain literal addresses outright. Gmail, Outlook, Yahoo, all of them. The reason is security. Domain literals bypass domain reputation checks, authentication (SPF, DKIM, DMARC can't verify an IP address the same way), and abuse tracking. If a spammer can send to captain@[192.0.2.44], they sidestep every deliverability safeguard built into modern email.
So if you're validating email addresses on signup forms or building an email list, reject any address with brackets around an IP. If you see one in your logs, it's either a test that should've been filtered or someone trying something weird.
You'll sometimes see domain literals in old SMTP server configs or technical documentation from the 1980s-1990s. Leave them there. For anyone sending real email today, this syntax is a historical artifact with no practical use.
Want to make sure your validation setup handles edge cases correctly? Check our guide on what counts as an invalid address, or see how catch-all addresses work if you're curious about other unusual address behaviors.
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