Do not open the Outlook software. Why open more than one email client?

Does Outlook use IMAP or POP to access the account?

The difference is that POP by default downloads the entire message from the server to your local disk and the usually removes it from the server. Thus, next time another e-mail client (or the same from a different location) tries to access the mailbox the message is gone.

In contrast, IMAP leaves the messages on the server which is the "master copy" of everything. E-mail programs only download the message as you read it, or synchronize their local "view" on the account with whatever happens on the server, thus every program has the same data (see this article for more information on the differences between the two protocols).

Thus, if Outlook has indeed grabbed all e-mails from your server, you'd have to put them back, which can only be done from Outlook. If you want to stick with the POP protocol, make sure to check something like "Leave Messages On Server" on all clients accessing your account, but then you'd have to clean up the messages yourself (e.g., using the provider's web interface) when the server is getting close to the maximum space allowed for you.