Qmail is a mail transfer agent (MTA) for Unix like operating systems. When I say MTA, naturally the question comes – what is an MTA? For that, first we need to know how internet email works.
Suppose Robert wants to send a mail to Ann. He opens his email client, enters the from field, types in the subject and matter. Once he clicks the send button, the mail is sent. Similarly Ann receives it at her mailbox. As far as they both are concerned, the system works fine, very well as desired. But behind the scenes, many small programs work together making it possible for the email to be moved from Robert’s machine to Ann’s.
MTA(Mail Transfer Agent) is the program which moves the mail from the sender’s system to the recipient’s system. It does this with the help of some other programs called Mail User Agents(MUA). The email client (for eg. Outlook Express, Thunderbird etc) with the help of which Robert and Ann send and received mails are called Mail User Agents. A user does not interact with his MTA directly. Instead he uses the MUA for it. A user uses his MUA to send mail, which hand over the mail to his Mail Transfer Agent. This MTA in turn contacts the recipient’s MTA. When the recipient pops his mail using his email client, the MTA transfers the mail to his MUA. This is how the mail system works.
In early days of Internet there were not much internet users and as such the security risk was less. In modern internet there are millions of users exploring internet daily, billions of cash transactions take place daily and sendmail can no longer support the security aspect. In 1995, Dan Bernstein, a mathematics student at the University of Columbia designed and implemented an MTA for modern internet and it was named Qmail. Qmail is just a built in replacement to sendmail.