Unable to open subdomain, alias or webmail hosted in Plesk: This site can’t be reached DNS_PROBE_FINISHED_NXDOMAIN

Symptoms

• WWW alias, website's subdomain or webmail URL that are hosted in Plesk cannot be opened with the following error:

  CONFIG_TEXT: This site can’t be reached
  DNS_PROBE_FINISHED_NXDOMAIN

  OR

  CONFIG_TEXT: This site can’t be reached
  ERR_NAME_NOT_RESOLVED

• The main domain example.com is accessible without any issues.

• The DNS checker https://mxtoolbox.com/ shows that DNS record for www.example.com, webmail.example.com or one.example.com is not published:

  

Cause

Missing DNS record (type CNAME) for www.example.com or (type A) for subdomain one.example.com or webmail webmail.example.com.

Resolution

Verify what name servers are used to serve DNS for example.com:

1. Go to the online NS record checker (for instance this one: Online tool for NS server check) and find what name servers are used for example.com;
2. Check if the NS servers are the same as in the Domains > example.com > DNS Settings (NS records). If they are, then DNS service is managed by Plesk.
   If different name servers are used, or the option Domains > example.com > DNS Settings is absent, 3rd party name server provider is used (i.e. DNS provider).