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!