When editing the MAILFILEDSPREFIX statement in hlq.SMTP.CONFIG, the name can have a maximum of 20 characters. The characters must be either alphanumeric, or one of the following symbols: period (.), dollar sign ($), number sign (#), or at sign (@), and the name must meet data set name conventions. The default name for this prefix, used in place of the incorrect name, is the job that is running SMTP. For more information, see the z/OS Communications Server: IP Configuration Reference.