Although Debian comes with Exim by default, the standard Unix MTA is Sendmail, and this is the standard for most other Linux distributions. If you are more comfortable with Sendmail, or have some other reason for using it instead of Exim, Debian will allow you to do this.

You can use the Debian package system to remove Exim, and install Sendmail in its place:

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As you can see, the Exim package has been removed, and the Sendmail one installed in its place. You will now be prompted to fill in the information necessary in order to configure the mail system, similar to what you've been required to do with Exim previously:

adduser: Warning: The home dir you specified already exists.
Saving current /etc/mail/sendmail.mc,cf to /var/backups

You are doing a new install, or have erased \
                /etc/mail/sendmail.mc.
If you've accidentaly erased /etc/mail/sendmail.mc, check
/var/backups.

Sendmail will not start until it is configured.
Do you wish to configure sendmail now, or wait until later?

Configure now ? (y/N) 
            

If you specify that you wish to configure Sendmail later, you'll be presented with the following:

To configure sendmail later, type sendmailconfig
After configuring sendmail, you can start it via \
                /etc/init.d/sendmail start
Press [ENTER] 

debian:~#