What causes “host not found”?

You send an email, and it bounces back with "host not found." Your first instinct might be that the address is fake. Sometimes that's right. But not always, and the difference matters a lot when you're deciding what to suppress and what to retry.

"Host not found" means your sending server tried to look up where the recipient's domain receives email (via DNS), and got nothing back. No address on record. No way to deliver.

The most common reasons this happens fall into two buckets: permanent and temporary.

Permanent causes (suppress these):

• The domain was never registered, so it genuinely doesn't exist
• The domain's registration expired and got dropped from the global DNS
• There's a typo in the address, like @gmai.com instead of @gmail.com
• The domain exists but has no MX records configured, meaning it was never set up to receive email

Temporary causes (worth retrying):

• DNS propagation is still in progress after a recent domain change (this can take up to 48 hours)
• The recipient's DNS servers are temporarily unreachable or overloaded
• A network issue between your mail server and the DNS resolvers it queries

Here's how to tell which bucket you're in. Start by checking the domain spelling. If "@gmai.com" should obviously be "@gmail.com", that's a typo. Suppress it and move on. Then check whether the domain is actually registered using a WHOIS lookup. If it shows as unregistered or expired, it's permanent. Suppress it.

Still if the domain looks legitimate and is registered, try querying its MX records directly using dig MX yourdomain.com or an online DNS lookup tool. No MX records returned means the domain was never configured for email. If you get MX records back but the bounce persists, you might be looking at a short-term DNS glitch worth retrying once or twice before suppressing.

If you're seeing a cluster of "host not found" bounces across multiple addresses on your list, that's a signal your list needs a clean. Expired domains and fake addresses tend to show up together when a list is old or was sourced carelessly. If that sounds familiar, our RME Clean service can sort them out before they drag your sender reputation down.