Retrieving your deleted email all depends on what email provider you have. If you deleted it off your phone it may still be in your inbox/deleted folder directly on your provider's website. So you'll need to log in to your provider's email service and check those folders to see if it's still there.
My suggestion: If you don't have one already, get a Gmail account. Even if you delete your email it will remain in the trash folders indefinitely, unless you delete it from there, but Gmail gives you so much storage space you don't ever have to delete emails. I have emails in there from a couple YEARS back.