Hello Burton_k,
Welcome to Apple Support Communities! I know it's important to be able to see all of your incoming emails, and I'll be happy to help out.
There are some mailboxes, such as Junk, which can contain an email that the email provider determined was junk or spam. Also check Trash for an email that was moved there before reading.
The smart mailbox, Unread, is a great way to be able to find an email that hasn't been seen yet. The iPhone User Guide, which has info for iOS 12 and iOS 13, has the steps to set that up. Look under the section titled "Organize your mail with mailboxes".
These steps should help you locate that unread email.
Cheers!