Removing trusted certificate from Mail in iOS 8 My IMAP (and SMTP) server used a self signed certificate.

Once I marked the certificate as "trusted" in the Mail app, is there a way to remove that trust again?

I tried deleting and re-creating the email account, but the certificate is still trusted afterwards.

The certificate does not show up in the Profiles section of the Settings app.
It looks like you might not have actually "trusted" it, but rather made "exception" for it as detailed here: https://blog.httpwatch.com/2013/12/12/five-tips-for-using-self-signed-ssl-certificates-with-ios/

If you did import the certificate, then the above website shows how to remove the "profile" that contains the certificate. To remove the exception, it looks like you need to reset all settings (General > Reset > Reset Settings). Unfortunately it deletes all your settings.

There is further discussion here:

How do you remove SSL certificate exceptions on iOS?