The short answer is not to use SNDDST for SMTP e-mails. It’s not designed not intended for it, though it can be used for some purposes.
Use a SMTP client if you need to send SMTP e-mail. There are a number of free utilities that you can download and install, as well as commercial products. Also, the system provides API access to SMTP facilities and to the entire mail server framework if you need special handling.
iSeries MIME & Mail is one commonly used free utility.