Hello dsherrill3,
We'd like to help with the error you're seeing on your iPhone.
Based on the information you have provided, it sounds like this error occurs in the Mail app.
If that is correct, ensure your iPhone is up to date: Update your iPhone, iPad, or iPod touch
If your iPhone is up to date, restart your device if you haven't already: Restart your iPhone
Although you mentioned this is not one of your mail accounts, how many mail accounts do you have set up on your device?
Do you have any VPN set up on your device? If so try disabling it and see if the issue persists.
Do you have any installed profiles or certificates? If so, try removing them: Install or remove configuration profiles on iPhone
If the issue persists, reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. Please know this also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you may have used before.
If the issue persists after all these steps, connect with Apple Support using this link so they can investigate further: Get Support
Thank you!