Can't remove request type used by email channel

I have a related question. I am setting up a new Service Management project and I want to change the default request type that is associated with the email requests (Request a New Feature) to be associated with another request type (Defect). Unfortunately, when I select "Configure email request type" the only option available in the drop-down is Request a New Feature.

I have confirmed that my request types are all set up correctly, so I'm not sure why they are not appearing in the drop-down for selection. This isn't critical, of course; I can change the request type once it has been submitted by email (which few of our clients do since they usually use the portal).

Thanks in advance if anyone has a suggestion.

Hello Terry, 

Just to check

From your service desk project, select Project settings > Email requests > Request type is set properly, to new request type (IT help)

You probably have old requests that are using the "old request type" you need to migrate them to "new request type, but if the request type is no longer associated with email request it should give you an option to remove the request type and migrate old issues to new request type.

BR, Olga