Hello Carrie711,
Thanks for posting in the Apple Support Communities. We're happy to look into this with you.
We've done a bit of research and found the only option is the one you've already attempted, which is to adjust the signature as shown here: Change email settings in Mail on iPhone - Apple Support.
Have you tried restarting your iPhone since you noticed the message returned? If not, you might want to give that a try to see if it resets the Mail app and corrects this to reflect your settings. The steps for restarting your iPhone are provided in article, Restart your iPhone - Apple Support.
If that doesn't work, you may want to contact Apple Support so they can take a closer look into this with you. You can reach them directly at Get Support.
Best regards.