Mail system explained

Sl.no	Agent	Description from Wikipedia
1	Mail 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.
2	Mail 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 itself.
3	Mail 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.
4	Mail 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 ravi(ravi@ravi.com) to his friend satish(satish@satish.com) will be delivered.


Step1:first ravi has to write a e-mail by specifying who is the receiver of this mail(in this case it is satish).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 ravi press send and receive in e-mail client,MUA initiate talk with its 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 ravi is sending 4 mails(one to satish@satish.com, one to prasad@satish.com, one to prahu@yahoo.com and one to raja@gmail.com) this MTA will segregate these 4 mails in to 3 pools(satish and prasad mails will be pooled in same pool so that they will be delivered to satish.com,prahu mail will be pooled to yahoo.com and raja's mail will be pooled to gmail.com)

Step4:Once the segregation has happened MDA(nothing but again its 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 satish and prasad mails has arrived to satish.com MAA) then this MAA will put the mails of satish and prasad 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).