sendmail is reliable. The program is designed to correctly deliver every message. No message should ever become completely lost.
sendmail uses existing software for delivery whenever possible. For example, the user interacts with a mail-generating and a mail-sending program. When mail is submitted, the mail-generating program calls sendmail, which routes the message to the correct mailers. Because some of the senders might be network servers and some of the mailers might be network clients, sendmail can be used as an Internet mail gateway. See Interactions of Mail Programs for a more detailed description of the process.
sendmail can be configured to handle complex environments, including multiple networks. sendmail checks the contents of an address as well as its syntax to determine which mailer to use.
sendmail uses configuration files to control mail configuration instead of requiring that configuration information be compiled into the code.
Users can maintain their own mailing lists. Additionally, individuals can specify their own forwarding mechanism without modifying the domain-wide alias file, typically located in the domain-wide aliases that are maintained by NIS or NIS+.
Each user can specify a custom mailer to process incoming mail. The custom mailer can provide functions such as returning a message that reads: “I am on vacation.” See the vacation(1) man page for more information.
sendmail batches addresses to a single host to reduce network traffic.