Hi mdme4,
I don't know whether it will work or not if you "un-spam" the original message ... maybe worth trying:
Depending on your settings ... deleted emails can often be recovered for up to 30 days:
You might want to check Trash or Bin or Archive mailbox-folders.
More info, if needed: Delete and recover emails in Mail on iPhone - Apple Support
or
There may be a copy in the Trash (or Archive) if you log-in to your account on on your eMail provider's website (eg: Yahoo / GMail etc.)
Else: If it is important to you ... there may be a copy in an iPhone back-up that you could restore.
Another tack: If you have another email address, you might just set up a new account with the app.
All the best :-)