The reason this is happening is because in order to add an email you need a BBIS “datasource”. The select export definition form actually creates this datasource behind the scenes and locks the export definition’s fields because it has an associated “datasource”.
Once an export definition is locked, it cannot be unlocked via the application. It could be unlocked with a SQL script, but the better option is to click the 'Copy' button and create a copy of the export definition. The copy will not be locked, and it can be edited and saved with a new name.