使用sudo apt install sendmail安装
结果如下：

sudo dpkg --configure -a
Setting up sendmail-bin (8.15.2-3) ...
Updating sendmail environment ...
Reading configuration from /etc/mail/sendmail.conf.
Validating configuration.
Writing configuration to /etc/mail/sendmail.conf.
Writing /etc/cron.d/sendmail.
Disabling HOST statistics file(/var/lib/sendmail/host_status).
Reading configuration from /etc/mail/sendmail.conf.
Validating configuration.
Writing configuration to /etc/mail/sendmail.conf.
Writing /etc/cron.d/sendmail.
Could not open /etc/mail/databases(No such file or directory),creating it.
Reading configuration from /etc/mail/sendmail.conf.
Validating configuration.
Creating /etc/mail/databases...
 
Checking filesystem,this may take some time - it will not hang!
  ...   Done.
 
Checking for installed MDAs...
sasl2-bin not installed,not configuring sendmail support.
 
To enable sendmail SASL2 support at a later date,invoke "/usr/share/sendmail/update_auth"
 
 
Creating/Updating SSL(for TLS) information
Creating /etc/mail/tls/starttls.m4...
Creating SSL certificates for sendmail.

卡在最后这一行，还有个进度条停在80%不动了。
解决方案：

cd /etc/mail/tls
sudo openssl dsaparam -out sendmail-common.prm 2048
sudo chown root:smmsp sendmail-common.prm
sudo chmod 0640 sendmail-common.prm
sudo dpkg --configure -a

解决方案出处：https://askubuntu.com/questions/937666/ubuntu-16-04-command-line-sendmail-installation-hanged/938450