Sending, receiving, and organizing email in Outlook.com
Hi. I'm Brian and I'll try to help.
I'll first state that if something needs to be retained for legal reasons, DON'T keep it in an online email service. Save it to local storage and keep more than one copy of it. Storage devices are cheap and provide a much safer means of making sure you have something you may need later.
How is your Sent Items sorted? Can I assume that you're using Outlook.com online, since that's the topic of this forum? Have you tried searching for the message at the mailbox level? Sometimes a missort can appear to hide a message.
If the message isn't in the Sent Items, Deleted Items or "Recover items deleted from this folder" folder, then it is gone and cannot be recovered.