What is an “unresolvable hostname”?

You've spotted the phrase "unresolvable hostname" in a bounce message, and now you're wondering whether this is your fault or the recipient's. Good instinct to ask. The answer changes what you do next.

A hostname is a human-readable name for a server, something like mail.example.com. For that name to actually reach a server, DNS has to translate it into a numeric IP address. An unresolvable hostname is one where that translation fails. DNS comes back empty. No address found. Connection can't be made, and the email bounces.

In email, this usually surfaces when the sending server looks up the MX record for a recipient's domain, follows the hostname listed there, and then tries to find the IP for that hostname. If the hostname in the MX record doesn't have a matching A or AAAA record in DNS, you hit this error. The MX record is there. The address it points to is not.

Is this your problem or theirs?

If the bounce is coming from emails you're sending, it's almost always the recipient's domain that has the broken setup. Their MX record points to a hostname that doesn't resolve. You can't fix that. You can try emailing them on a different address, or let the relationship owner know their mail server has a DNS issue.

It's your problem if you're the one receiving this error about your own sending domain. That can happen if you recently migrated mail servers, changed DNS providers, or updated your MX records but forgot to add the corresponding A record for the new hostname.

Common causes

• A typo in the MX record hostname (like mail.examlpe.com instead of mail.example.com)
• An MX record pointing to a hostname that has no A or AAAA record at all
• A hostname that was deleted from DNS but never removed from the MX record
• DNS propagation delays where a new record hasn't fully spread yet

How to check it yourself

Start by looking up the MX records for the domain in question. You can use a free tool like MXToolbox or run a quick terminal lookup with dig MX yourdomain.com. You'll see the hostname your MX record points to.

Then look up that hostname directly. Run dig A mail.yourdomain.com (or use MXToolbox's A record lookup). If you get an empty result or NXDOMAIN, you've found your problem. The hostname exists in the MX record but has no IP address attached to it.

Still the fix is to either add the missing A record for that hostname, or correct the MX record so it points to a hostname that actually exists and resolves.

If you're looking at a DNS-related bounce in a report and aren't sure whether it's yours to fix, our email header analyzer can help you read through the delivery trace. Or if this is actively breaking deliverability, our SOS hotline is free and we actually look at your setup with you.