At a glance, the issue is minor. There are solutions available on the internet. For example, this
(1) How To Install and Configure Postfix as a Send-Only SMTP Server on Ubuntu 14.04 at https://www.digitalocean.com/communi...n-ubuntu-14-04
(2) Simple local smtp server - minimal setup ubuntu on https://superuser.com/questions/3472...l-setup-ubuntu
Requires postfix.
Postfix requires following dependencies, in chained order of pre-requisite:
postfix_2.9.6-2_i386.deb
mailutils_2.99.97-3_i386.deb
mailutils-common_2.99.97-3_all.deb
libmailutils4_2.99.97-3_i386.deb
libgsasl7_1.8.0-2_i386.deb

Finally, attempt to installation of postfix reports:
"clashes with smail"

I skimmed through the man page of smail.
Quote Originally Posted by man smail
... It is not intended to be a user interface for
reading and submitting mail. See mailx(1) on System V, Mail(1) on BSD
systems, or mail(1) on other systems for information on user inter‐
faces ...
I tried this command:
Code:
smail -bP '${smtp_hello_reject_hostnames}'
Elaborate output more than just the intended line in man page of smail:
Code:
localhost(\.)*;You've got to be kidding!  If you really were 'localhost' I'd be talking to myself!:.*\.localdomain;There is no such domain 'localdomain'!:.*\.test;There is no such domain 'test' -- it is reserved by IANA as per RFC 2606:.*\.example;There is no such domain 'example' -- it is reserved by IANA as per RFC 2606:.*\.invalid;There is no such domain 'invalid' -- it is reserved by IANA as per RFC 2606:.*\.localhost;There is no such domain 'localhost' -- it is reserved by IANA as per RFC 2606:(.*\.)*example\.com;There is no such domain 'example.com' -- it is reserved by IANA as per RFC 2606:(.*\.)*example\.net;There is no such domain 'example.net' -- it is reserved by IANA as per RFC 2606:(.*\.)*example\.org;There is no such domain 'example.org' -- it is reserved by IANA as per RFC 2606:${rxquote:hostnames}
Skimmed through man page of mail:
...
Sending mail
To send a message to one or more people, mail can be invoked with argu‐
ments which are the names of people to whom the mail will be sent. You
are then expected to type in your message, followed by a control-D (‘^D’)
at the beginning of a line. The section below, Replying to or
originating mail, describes some features of mail available to help you
compose your letter.
...
The /etc/mail.rc contains the lines:
set ask askcc append dot save crt
ignore Received Message-Id Resent-Message-Id Status Mail-From Return-Path Via Delivered-To
Easily understood.

A visual editor could also be used:
EDITOR Pathname of the text editor to use in the edit command and
~e escape. If not defined, /usr/bin/ex is used.
Now to exim4. Uses MTA. Details in https://packages.debian.org/wheezy/exim4
But also clashes with smail.
I am not amenable to remove something that is installed by Knoppix system developers.

I won't like to use the "mail" command, as it involves a lot of editing the system files and isn't very intuitive, one which is an automatic plus for visual email clients.

I have to set up a local smtp email server first through the tools available within Knoppix. Then, to use icedove or claws-mail. Without installing postfix as it clashes with smail.

I know 7.2.0 is an ancient version. But it works perfectly okay for me. So I don't wish to replace it. Doesn't require replacement at least, in the present case.