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What the error actually means
SMTP does not deliver mail to a website. It first asks the Domain Name System (DNS) which host accepts mail for the recipient’s domain. Mail eXchanger (MX) records identify the next-hop destination for a message. Postfix cannot begin an SMTP connection when the MX lookup fails, returns unusable data, or when the address lookup for the selected host also fails.
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The phrase may appear as Sorry, I couldn't find a mail exchanger or IP address. (#5.4.4). The numeric response, 5.4.4, is a persistent delivery failure commonly associated with an unreachable destination. The exact sentence is also used by qmail-style bounce diagnostics and by other tools, so wording alone does not prove which mail-transfer agent generated the bounce.
There are two separate questions in this failure:
- Can DNS identify a destination? The recipient domain needs usable MX information, or a usable address under the applicable SMTP fallback rules.
- Can Postfix reach and use that destination? A host can resolve correctly and still refuse a connection, reject a recipient or defer service. Those are later SMTP-stage failures, not the original DNS error.
Why a working website does not disprove an email problem
Web and mail services use different DNS records and can run on different hosts. A browser may successfully retrieve https://example.com because the domain has an A or AAAA record for its web server, while the domain has no MX record or has an MX target that cannot be resolved. HTTP success therefore says nothing conclusive about SMTP routing.
Test the complete recipient domain, including its spelling and top-level domain. A typo after the @ sign can produce the same bounce as a misconfigured zone. If the domain does not exist, or belongs to a different organization, changing Postfix will not fix the address.
Use this order of operations
Work from the recipient outward. Public DNS must be correct before local routing overrides are considered.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errors- Confirm the recipient address and isolate the domain after the @ sign.
- Query that domain’s MX records.
- Resolve every MX target to A and AAAA records.
- Check that no MX target is a CNAME alias.
- Compare the authoritative zone with independent recursive resolvers.
- Inspect non-default Postfix settings without making speculative edits.
- Retry one message and read the resulting queue and SMTP log entries.
Step 1: Confirm the recipient domain
Check the address itself
Read the address character by character. Look for a misspelled organization name, an extra period, a missing character, or a country-code ending that was replaced with a different one. Copy the domain without the local mailbox portion and use it in DNS tests.
Do not infer that a domain is valid because it opens in a browser. Your first DNS test should be the exact domain used by the recipient address.
Step 2: Query MX, A and AAAA records
Run the basic queries
From a shell, replace the example names with the recipient domain and each MX host returned:
dig MX example.com
dig A mx.example.com
dig AAAA mx.example.comOn systems without dig, equivalent queries can be made with:
nslookup -q=mx example.com
nslookup -q=a mx.example.com
nslookup -q=aaaa mx.example.comThe MX response should contain one or more preference values and host names. Lower preference numbers are selected before higher numbers when multiple destinations are available. Record every target, not only the first one; a later target may be the only usable destination if the preferred host is down.
Interpret the results
- No MX answer: Determine whether the domain is supposed to receive mail. If it is, inspect the authoritative zone for a missing record. SMTP can apply address-record fallback when no MX exists, but that fallback still requires a usable A or AAAA address and does not repair a nonexistent domain.
- MX names are returned, but a target has no A or AAAA answer: The destination name cannot be used to open an SMTP connection. Add the required address record to the target host or correct the MX value.
- An error such as a nonexistent domain appears: Recheck the recipient spelling and the domain’s authoritative DNS. Do not edit Postfix until the domain itself is confirmed.
- Different resolvers return different answers: Compare them with the authoritative server. A recent correction may be visible at one resolver while another still serves its cached answer.
Check consistency, not just one response
A single successful lookup from the sending server is not enough when users report intermittent bounces. Query the domain through the resolver used by Postfix and at least one independent recursive resolver. Confirm that the MX preference values and target names are consistent and that each target resolves repeatedly to at least one usable address.
Step 3: Fix an MX target that is a CNAME
Why the alias is invalid
An MX record’s value must not be a CNAME alias. For example, this design is invalid:
example.com. MX 10 mail.example.net.
mail.example.net. CNAME mail-provider.example.Although a normal application lookup may follow the alias, SMTP’s MX rules require the mail exchanger name itself to be a canonical host name with address records. Depending on resolver behavior, an alias target can produce a delivery failure even when the aliased name ultimately points at a server.
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Publish the canonical host name directly in the MX record and give that host an A and/or AAAA record:
example.com. MX 10 mx.example.net.
mx.example.net. A 192.0.2.25If IPv6 is intended, publish an AAAA record as well. If IPv6 is not ready for SMTP, do not publish an AAAA address that is unreachable; every address returned for the target should be usable by the sending infrastructure.
Make this change in the authoritative DNS control plane, not in a local hosts file. A local workaround may hide the problem for one sender while other mail systems continue to fail.
Step 4: Verify authoritative DNS and cache propagation
Inspect the authoritative zone
DNS administrators maintain the authoritative master data for a domain. Use the registrar or DNS provider’s zone editor to confirm:
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- The MX record is published at the exact recipient domain.
- Each MX target is spelled as a fully qualified host name in the intended zone.
- Every target has the required A and/or AAAA record.
- No target name is an alias created with CNAME.
- There is no accidental record at a similar-looking subdomain.
When checking from a shell, first identify the authoritative name servers for the domain, then query one directly. Compare that answer with the answer from your normal recursive resolver. The authoritative response tells you what the zone currently publishes; the recursive response tells you what a cached client is currently seeing.
Allow for cached data
Resolvers cache DNS answers and refresh them according to the zone’s configured policies. Consequently, a corrected MX record can appear at the authoritative server before all recursive resolvers stop serving the older result. This is normal DNS behavior, not evidence that Postfix ignored the correction.
Do not repeatedly edit the zone while caches are still converging. Record the time of the change, check authoritative data first, and then sample independent recursive resolvers until they return the corrected MX and address records. The exact interval depends on the previous records’ cache policies.
Step 5: Inspect Postfix without creating a second problem
Review effective settings
Postfix’s main.cf file contains parameters controlling the mail system. Debian’s postconf(5) documentation specifically warns that unnecessary parameter changes can impair operation. Inspect before editing:
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postconf -n
postconf relayhost
postconf transport_maps
postconf mydestinationpostconf -n lists non-default settings. The individual postconf parameter form shows the effective value of one parameter. Save the current output so a justified change can be reversed.
Know which settings matter
relayhost: Determines whether mail is intentionally handed to another SMTP service instead of being delivered directly to recipient MX hosts. An unintended value can route mail somewhere other than the path you are troubleshooting.transport_maps: Can override normal destination selection for particular domains. Check for a stale or misspelled map entry that sends this recipient domain to an invalid host.mydestination: Defines domains Postfix treats as local. It is relevant when the server believes it is the final destination, but changing it merely to silence a bounce can misroute mail.- Resolver-related settings: Verify that the server can reach its configured DNS resolvers and is not using an obsolete local resolver configuration. A local resolver failure can make a valid public zone appear broken only to this Postfix host.
Change only the value required by the intended mail architecture. If independent resolvers cannot find the recipient’s MX records, fix DNS rather than adding a Postfix transport override. If public DNS is correct but this server alone selects the wrong route, investigate the local map or relay design.
Step 6: Retry and distinguish the next failure
After correcting DNS and allowing resolver caches to update, retry one message. Watch the Postfix log for the selected MX host, the connection result and the final SMTP response.
- Successful lookup followed by a connection refusal or timeout: DNS is working; investigate destination availability, firewall policy or network reachability.
- Connection succeeds but the recipient is rejected: The remote server is reachable and has moved to recipient validation or policy checks. Read its SMTP response; do not treat it as an MX failure.
- Delivery is deferred: Postfix may retry according to its queue policy. Check whether the log names a temporary DNS or connection condition.
- The same 5.4.4 error returns: Capture the exact domain and MX response seen by the sending server. A resolver cache, an uncorrected MX target or a transport-map override is still selecting an unusable destination.
Test with one recipient after each material correction. Sending a large batch while DNS is still inconsistent makes it harder to tell which messages used old answers.
Choose the right repair path
| Observed condition | Most likely owner | Preferred action | What not to do |
|---|---|---|---|
| Independent resolvers cannot find MX or usable address data | Recipient-domain DNS administrator | Publish or correct the authoritative MX and target A/AAAA records | Do not mask a public DNS fault with a sender-only route |
| Authoritative DNS is correct, but some recursive resolvers show old data | Resolver cache and DNS timing | Wait for cache refresh while monitoring authoritative and recursive answers | Do not keep changing records to chase each cached response |
| MX points to a CNAME | Recipient-domain DNS administrator | Use the canonical host name in MX and publish its A/AAAA record | Do not rely on alias-following behavior |
| Public DNS is correct, but only one Postfix host fails | Sender-side configuration or resolver | Inspect resolver access, relayhost, transport_maps and effective settings |
Do not rewrite unrelated main.cf parameters |
| Mail architecture intentionally uses a smarthost | Sender-side mail administrator | Verify the approved relayhost and its own DNS and connectivity | Do not force direct-to-MX delivery if policy requires the relay |
Common scenarios and precise fixes
The domain has a web server but no mail service
Some domains are intentionally web-only. If the recipient domain should not receive mail, use the correct mailbox domain. If it should receive mail, its administrator must publish a mail destination and address records; the website’s A record is not a substitute for deliberate mail configuration.
Only one MX target is broken
Multiple MX records provide ordered alternatives, not a guarantee that every target is valid. Resolve each target individually. Repair or remove a dead target and verify the remaining preference order in the authoritative zone. Do not assume that a working low-priority target makes an invalid preferred target harmless; sending behavior can vary while the preferred host is attempted first.
The error started immediately after a DNS migration
Compare the new authoritative name servers and their zone contents. A registrar change can delegate a domain to servers that do not contain the previous MX records. If the new authority is correct but public resolvers disagree, cache timing explains the transition; if the authority itself lacks the records, restore them there.
Only this Postfix server reports the error
Query MX and target addresses from the affected host. If those results differ from independent resolvers, inspect its resolver configuration and network access to DNS. Then review postconf -n for transport or relay overrides. A sender-local failure should not be “fixed” by altering the recipient’s DNS without evidence.
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Do not identify the generator from the sentence alone. qmail-style diagnostics and other systems can use similar wording. Use the complete bounce headers, the sending Postfix log and the DNS tests to determine which hop produced the failure.
Prevent the error from returning
- Keep MX records and their target A/AAAA records in the same documented DNS change process.
- Review every MX target after a provider, registrar or hosting migration.
- Reject CNAME targets during DNS review; an MX value must be canonical.
- Monitor authoritative and recursive answers after planned changes, with the domain and record type recorded.
- Keep a copy of effective Postfix settings and document any intentional relayhost or transport-map route.
- When troubleshooting, save the exact DNS responses and log lines with timestamps so cache changes can be distinguished from configuration changes.
When to hand the issue to the domain administrator
If the recipient domain is not yours, you cannot repair its authoritative MX records. Send the domain owner the exact findings: the queried domain, the MX response, which target lacked A/AAAA data or used a CNAME, and whether the authoritative server differed from recursive resolvers. That evidence is more actionable than a generic “email is broken” report.
If the domain is yours and authoritative DNS is correct, give your mail administrator the affected recipient, the resolver answers seen by Postfix, the output of postconf -n with sensitive values protected, and the relevant log entry. This separates a public-zone problem from a local routing or resolver problem without risking speculative edits.
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