Although this version of sendmail provides many new features, the –FallBackSmartHost option is the most significant addition. Because of this option you no longer need to use main.cf and subsidiary.cf. The main.cf file was used in environments that supported MX records. The subsidiary.cf file was used in environments without a fully operative DNS. In such environments a smart host was used instead of MX records. The –FallBackSmartHost option provides unified configuration. It operates like an MX record of last possible preference for all environments. To ensure that mail gets delivered to clients, this option, if enabled, provides a well-connected (or smart) host that serves as a backup (or failover) for MX records that fail.
For more information about version 8.13, see the following sections:
Additional and Revised Configuration File Options in Version 8.13 of sendmail
Additional and Revised FEATURE() Declarations in Version 8.13 of sendmail
Additionally, SMTP can run with Transport Layer Security (TLS). See the following description.