Here I just choose postfix. Someone on the Internet said the configuration was very complicated, but after I install postfix, I found the default config file is OK if you just want to send email from you locathost.
sudo apt-get install postfix
At first time when you test sending email, you might encount with such error:
postdrop: warning: unable to look up public/pickup: No such file or directory
That's because old sendmail daemon still running, just stop it and restart postfix can fix:
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$ sudo /etc/init.d/sendmail stop $ sudo mkfifo /var/spool/postfix/public/pickup $ sudo /etc/init.d/postfix restart