Sounds like you have postfix installed and running.
You should check your postfix configuration.
There is a good description here.
Or do it your self:
If postfix is still not working, check the log file in /var/log/mail.log for the error:
sendmail: fatal: chdir /Library/Server/Mail/Data/spool: No such file
or directory
To fix this, run the following commands in Terminal:
sudo mkdir -p /Library/Server/Mail/Data/spool
sudo /usr/sbin/postfix set-permissions
sudo /usr/sbin/postfix start
If as you say you do not want it to be on your computer then disable it.
sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/org.postfix.master.plist
That does not answer your question who is using it, but without more info from you one can not say, it could be a script in your system.
You could try to find out who is sending by going to
Open your Console:
Scroll down to Var/Log/mail.log
The background: Mac OSX comes with the postfix MTA, which is a fully
featured SMTP server. Under normal circumstances, there is usually no
need to enable or configure this software, as most email access is
usually done via GUI clients such as the Mail.app – which uses the
POP/IMAP and SMTP settings to connect with the email service provider.
However, there are certain circumstances in which having a local SMTP
server is very useful, such as: Allowing the batch logs and output
from the cron daemon or other scripts to be sent via Internet email
(this is otherwise delivered locally) Testing email based code; which
requires a local sendmail like SMTP server to be present
