SMTP can be configured to operate without DNS, using IP addresses instead of domain names for SMTP clients, SMTP servers, and recipient email addresses. However, this configuration is rare.

SMTP as it is typically used relies upon DNS to determine the mail gateway server (MX) for a domain name, and to resolve domain names into IP addresses. As such, you usually must configure email servers and FortiMail units to be able to query a DNS server.

In addition, you may also be required to configure the DNS server with an MX record, an A record, and a reverse DNS record for protected domain names and for the domain name of the FortiMail unit itself.