apt-get install mail-stack-delivery
Reading package lists... Done
Building dependency tree
Reading state information... Done
mail-stack-delivery is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 145 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n]
Setting up mail-stack-delivery (1:2.2.18-2ubuntu2) ...
grep: /etc/dovecot/conf.d/10-ssl.conf: No such file or directory
grep: /etc/dovecot/conf.d/10-ssl.conf: No such file or directory

Postfix not configured. Run
sudo dpkg-reconfigure postfix and choose
the type of mail server. Then run
sudo dpkg-reconfigure mail-stack-delivery to
finish mail-stack-delivery installation.