I was setting up the email to case, under Service Management\Automatic Record Creation and Update Rules, in the Specify Record Creation And Update Rule Item, I had added step to create case.
And I had encountered in the Case form, that one of the field is greyed out, this field is not readonly but a locked field and is a customised text field, properties: Data Type: Single line of Text, Field Type: Simple, Format: email.
Any clues? Thanks.