Safe to delete email messages after import?

I am keeping messages from an academic list that has grown huge as an archive in my email clients; moreover, part of them are in Entourage and part of them in Mail. So I have thought the best way to archive them, to fuse them in a single database and to search the content easily is to create a database with DT Pro Office.
I am in the process of importing some 6,000 email messages, and thousands of them will be added to the database over the next few years.
Simple question: once I have imported them, and made a backup of my Mac, is it safe to delete the original email messages and only to keep them in the DT database? Or do you suggest a different archiving strategy?

I would suggest making a separate archive as a second backup. In Apple Mail use Mailbox > Archive Mailbox. This creates an mbox file that you can compress and store safely, and restore to Mail if needed.

Thank you, this is indeed a good idea for additional safety. I will certainly follow your suggestion.
Can something similar be done with Entourage?

I’ll add to the Apple Mail suggestion that you’ll need to archive each mailbox separately (Inbox, Sent, and any other mailboxes that the account might have). I’ll leave Entourage to someone else.