Hello there, @Kelin.
Allow me to share a few information about the attached documents in QuickBooks.
The attached document will not be deleted or disappeared when you delete the email from the outlook. However, this attached document will be saved in the Attach folder where the company file located.
Feel free to read this article to learn more about attaching documents: QuickBooks Document Center: FAQs and Common Issues.
Stay in touch with us if you have other questions about attachments in Quickbooks. The Community team is always around to help.