After I have activated the Outlook indexation, only new emails appear as search results. Old emails seem to be not included in the index.
When you say "old emails", how old approximately? Are they stored in a separate PST archive file? And when you say "only new emails", i guess you only have emails that you received after activating your Outlook indexing?
The index may not completed all your email folders yet.
Please try the following:
Let us know how it goes!