The email I entered for claiming ownership does not match the email in the RSS feed.
Please note that claiming ownership only applies to podcasts that are hosted elsewhere. If your content is hosted on Castbox, you already have ownership of your channel.
One possible reason for receiving the error message is that an auto-generated email address could be displayed in your RSS feed, especially if your podcast is hosted on Anchor, Audioboom, Pippa, or SoundCloud.
To check what the email address is displayed in your RSS feed, you can open the RSS URL and search for <itunes: email> tag in the Google Chrome desktop browser.
You can update this email address using your hosting provider's service. Here are some useful references about how to update the email address for different hosting sites.
Anchor: Please contact Anchor's support for assistance.