使用sudo apt install sendmail安裝
結果如下：

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.
 

卡在最後這一行，還有個進度條停在80%不動了。
解決方案：

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