Why does mailutils depend on mariadb-common and mysql-common and what are *-common packages in the first place?

Solution 1:

postfix has lots of features. Dependency chain: postfix, postfix-mysql, libmariadbclient18, mysql-common.

To skip optional dependencies, use apt-get --no-install-recommends.

As to what "common" packages, are from mysql-common description:

This package includes files needed by all versions of the client library, e.g. /etc/mysql/my.cnf.

Extrapolating a bit, common packages contain files used by most users of a thing. They might be shared libraries containing common code, where a server and a client package can be installed separately. Or, shared config files used by all versions of a thing.