Best way to backup mails at work (Outlook Mac 2011)

[hajime]

Hello, people at school usually use Outlook (standalone program or via the web) to access emails. I use Outlook Mac 2011. Recently, I often get the mailbox is going to full message. Even after I have used the archive option, I get the mailbox is going to full message in a week. Somebody suggested that I delete some big files. However, I want to keep ALL of them for record. What is the best way to backup ALL the mails? Thanks.