When an email client cannot resolve the server hostname, it may show messages such as “server not found”, “cannot resolve hostname” or “DNS lookup failed”. This means that the email client cannot locate the mail server named in its account settings. The problem is usually related to the server hostname, DNS, the local network or a service outage. It is different from an incorrect mailbox password or an SMTP login error.
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What does “cannot resolve server name” mean?
A mail client such as Outlook, Apple Mail or Thunderbird connects to a mail server by using a hostname. DNS is the system that matches that hostname to the server address needed for the connection. If DNS cannot return an address, the client cannot connect to the mailbox.
The wording can vary between applications. “Server not found” and “cannot resolve hostname” describe the same type of failure: the client cannot locate the server named in the account configuration. “DNS lookup failed” identifies DNS as the part of the connection that did not complete.
Hostname errors and login errors are different
| Error type | What it means | What to check first |
|---|---|---|
| Hostname or DNS error | The client cannot locate the mail server. | Server name, DNS availability and network access. |
| SMTP authentication error | The client reached the outgoing mail server but did not authenticate correctly. | SMTP login and outgoing server authentication. |
An error such as “sender address rejected: not logged in” is not a DNS error. It appears when the email application tries to send without properly authenticating to the outgoing mail server. This can occur in Outlook, Thunderbird, Apple Mail and similar applications. Read more about the “sender address rejected: not logged in” error.
How to diagnose the connection
- Read the exact error. Confirm that the message refers to a missing server, hostname, or DNS lookup. If it refers to authentication, the issue belongs to the login settings instead.
- Check the hostname character by character. Compare the incoming and outgoing server names with the settings supplied for the mailbox. Check for missing characters, extra spaces, a wrong domain ending, or a hostname entered in the wrong field. A single character can make DNS look for a different server.
- Check both mail-server fields. Receiving mail and sending mail can use separate server settings. If only sending fails, check the outgoing server hostname. If only receiving fails, check the incoming server hostname. If both fail, check the account’s network access and DNS first.
- Test the same account from the current network. Check whether the error occurs only on one device or email client. If another device on the same network shows the same error, the network or DNS connection may be involved. If the problem appears only in one application, review that application’s account settings.
- Check for VPN or firewall interference. A VPN, local firewall, or network security filter can affect DNS requests or block access to a mail server. If your network policy allows it, test the connection with the VPN disconnected. A firewall test should be temporary, followed by restoring the normal protection settings.
- Check whether DNS is working for other services. If websites and other services also fail to load by name, the issue may affect the local network or its DNS service rather than the mailbox. If other services work but the mail hostname does not, recheck the mail hostname and contact the email provider if the name is correct.
- Allow for a provider-side interruption. If the hostname is correct, the network can resolve other services, and the problem affects more than one client or device, the mail provider may need to check the service. Provide the exact error, the affected mail function, and the time the problem started.
Which settings can be changed safely?
Changes to the incoming server hostname, outgoing server hostname, DNS connection, VPN state, or firewall test affect how the client connects. They do not change the mailbox content. Record the original values before editing them so they can be restored if needed.
Change one connection setting at a time and test after each change. Do not treat a DNS error as a reason to remove the mailbox account or delete messages. If the error is actually an SMTP authentication error, focus on the outgoing server login instead of changing DNS. SMTP error codes can help identify the reason a message was not delivered. See the SMTP error code explanations.
When to contact the email provider
Contact the provider when the hostname matches the supplied settings, the issue remains across more than one client or network, or the same server cannot be reached after local DNS and network checks. Include the exact hostname, the full error text, whether sending or receiving is affected, and whether the problem occurs in Outlook, Apple Mail, Thunderbird, or another client.
If new messages have also stopped appearing, review the email client settings as part of the diagnosis. Use the mailbox message troubleshooting steps.