AIX - Sendmail

I would like a good overview on sendmail:

What sendmail is?
What is it used for?
Where do you set it up?

Please provide a good explanation and simple basic help links.
Who is Participating?
"sendmail" is basically a background process providing (under AIX in cooperation with "bellmail") mailserver functionality (accept incoming mail, deliver to recipients, act as a relay or mail hub etc.).
So there are MTA (Mail Transfer Agent, sendmail) and MDA (Mail Delivery Agent, bellmail(AIX)) functionalities.

Instead of using AIX sendmail's standard MDA "bellmail" you can use other ones like "procmail".

sendmail's MTA is used to accept incoming mail messages, bellmail's MDA  is used in AIX to deliver them to the mailboxes of local recipients and sendmail uses the SMTP protocol to act as a relay in order to forward the messages to other mailservers, according to settings in its own config or in DNS (MX).

Local configuration is done in /etc/mail/
Under AIX this is traditionally done by modifying the file directly, in many other OS the macro language "m4" is used to generate the final configuration out of macro definitions.
This is also possible in AIX, but not widely used, as I already mentioned above.

Please contrast the sendmail server daemon with end user interfaces like "mail", "mailx", "pine", "mutt", "elm" etc.
These are called  MUA (Mail User Agent) and are used to compose and send messages or to manage the mailbox.
Mail clients use or their own config files to get infrastructure information. They don't rely on a running local sendmail daemon (MTA), but there must be an MTA somewhere else in the network they're aware of.

A tutorial:

AIX Infocenter:
AIX25Author Commented:
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