Send mail is one of the old and widely used mail delivery agent/Mail system.

Sl.noAgentDescription from Wikipedia
1Mail User Agent(MUA)An e-mail client (also mail user agent (MUA) or e-mail reader) is a frontend computer program used to manage e-mail.
2Mail Transfer Agent(MTA)A mail transfer agent (MTA) (also called a mail transport agent, message transfer agent, or smtpd (short for SMTP daemon), is a computer program or software agent that transfers electronic mail messages from one computer to another.The term mail server is also used to mean a computer acting as an MTA that is running the appropriate software. The term mail exchanger (MX), in the context of the Domain Name System formally refers to an IP address assigned to a device hosting a mail server, and by extension also indicates the server it’self.
3Mail Delivery Agent(MDA)A Mail delivery agent (MDA) is software that delivers e-mail messages after they have been accepted on a server, distributing them to recipients’ individual mailboxes.
4Mail Access Agent(MAA)A way of remotely accessing a mail-spool, for example POP3 or IMAP.

A short description how a Mail communication will work:

Here I will take an example how a mail from John( to his friend Jelo( will be delivered.

Step1:first John has to write a e-mail by specifying who is the receiver of this mail(in this case it is Jelo).So  things like composing mails will be taken care by MUA.this MUA( A mail client such as MS outlook,Thunder-bird etc) knows how to deals with mail transportation,Once Jhon press send and receive in e-mail client,MUA initiate talk with it’s MTA(A SMTP server which is listening  on 25 port for receiving mails).

Step2:Once MTA(such as SEND-MAIL,Qmail,Post-Fix etc) get the request from MUA, MTA will take care of all the activity such as communicate with MUA and fetching mails from MUA etc.

Step3:Once MTA got the mails from MUA, it has to segregate all the mails according to different domains. for example Jhon is sending 4 mails(one to, one to, one to and one to this MTA will segregate these 4 mails in to 3 pools(jelo and jennie mails will be pooled in same pool so that they will be delivered to,anne mail will be pooled to and raja’s mail will be pooled to

Step4:Once the segregation has happened MDA(nothing but again it’s a send-mail which will take care of )will delivered these mails to respective MAA(such as Dovecot ).

Step5:Now suppose mail has delivered to their respective MAA’s. Once the mail reaches, MAA will try to segregate the mails according to users(here for example Jelo and Anne mails has arrived to MAA) then this MAA will put the mails of Jelo and Anne in respective mails boxes in a Mail Store(MS) as shown in Figure.

Step6:Now MUA will come in to picture in downloading these mails from MAA, so MUA will communicate with MAA and download the mails to MUA(like outlook and thunder-bird).

Please comment on this and we will post basic configuration of Sendmail in my comming posts.

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