Database Design Question

I was sitting down thinking the other day about when is it good to
generate a new database or just use an existing one. For example, lets
say my company name is called 'databasedummy.org' and I have a
database called 'dbdummy'. Now I need PostgreSQL to manage several
applications for my company:

- webmail
- software
- mediawiki
- phpbb forum

Now what I've been doing is just creating multiple tables in the
'dbdummy' database but each table is owned by different users
depending on their role. Is this bad? Should I be creating new
databases for each application above rather than one single company
database?

Just trying to understand good DBA design practice. This is obviously
a very general question but any feedback on what good or bad issues
would come from me dumping all my tables for applications in one
database or spread out across multiple databases on PostgreSQL.

Thank you!