Sendmail is one form or type of implementation of the simple mail transfer protocol, or SMTP, that is common with the use of electronic mail services today. Mail servers make use of the sendmail function by attempting to deliver electronic mail messages in real time. In the event that the recipient is not readily available to receive the inbound mail, the messages are placed in a queue until it is possible to complete the delivery. While sendmail is a general term used to describe this type of interface, there is also a commercial product that is known as Sendmail that incorporates this and other features into the overall functionality.
It is important to note that sendmail focuses on the delivery component of the inter-network email routing facility. The server works in tandem with several other components in order to allow the end user to enjoy full access to mail delivery along with the ability to send original emails or reply to received emails. This means that a mailbox facility must be designated as the point of termination for the email transmission, and that the mailbox must also be identified with a POP3 or IMAP server in order to send emails. Providers of both free and fee-based email services supply their customers with the information needed to provide the full range of services to end users, making it possible to send and receive email with ease.
While not the only means of transmitting email, sendmail does have a history that dates back prior to the advent of widespread access to the Internet. The strategy is considered a direct descendant of the deliverymail program that was developed in the latter part of the 1970s and was later released as open source software. Over time, deliverymail was incorporated into both free software and various types of proprietary programs for electronic mail. As with most other types of software related to emails, sendmail has occasionally been updated into newer versions that seek to correct flaws uncovered in earlier versions.
The number of users of sendmail as a mail delivery agent have declined since the beginning of the 21st century. While continuing to be one of the more popular alternatives, other agents have come into regular use as consumers continue to seek the most efficient and most secure means of setting up email networks. As a commercial product, Sendmail continues to release newer versions that will allow this particular approach to compete with other Internet messaging platforms in the years to come, and possibly recapture a larger portion of the market.