Reference
Configuration of the DNS server address in the "DNS configuration" parameter group
A DNS server may be required when the module itself, a communications partner, or for example an e-mail server should be reachable via the host name (FQDN).
DNS server for e-mail connections
When configuring an e-mail connection, the address of the e-mail server via which the e-mails are sent must be specified. The address of the mail server can be specified as an IP address or as an FQDN.
If you specify the server address as an FQDN you need to configure a DNS server. In this case the IP address of the mail server is determined via the configured DNS server.