You might or might not be able to send a particular file. It depends on what kind of file and what kind of e-mail you are working with.
First, if it’s a SNA network, then you can send almost any kind of file. That’s what SNDDST is intended for.
But if it’s SMTP e-mail over TCP/IP, then you have to follow the rules of SMTP. Unless it’s a simple text file, you will have a lot of work to do before you can send it. SMTP only allows text files.
In order to send non-text, you need to encode it as one of the common MIME types. And actual sending should then be done with the QtmmSendMail APIrather than SNDDST. There are also a few SMTP utility programs for AS/400s that can be found through internet search engines; download one of those to help make things easier.
You should use SNDDST only as a simple way of sending basic messages — plain text e-mails.