Ubersmith has three methods of sending outbound messages:
PHP Internal Mail
This uses PHP's internal mail() function, and is the default -- and preferred choice. No additional configuration is necessary.
This option uses the Sendmail binary at the operating system level. Sendmail (or another compatible MTA, such as Postfix) must be installed to take advantage of this feature.
This option sends outbound mail via an external SMTP server. You must provide the Hostname or IP address of the SMTP server, as well as a valid username and password. This is the least reliable of the three methods, as it requires the connection, as well as the authentication, between the two hosts to always be functional. However, if you want to have outbound mail pass through an external server first, this is the way to do it.