Try enclosing the file name in single quotes. I don’t use the SNDDST command so this may not work.
My advice to you is to dump SNDDST and go with an email program that was created for the iSeries. Two of the free ones that come to mind are MMAIL and RPGMAIL. There are also a host of commercial programs available. We use MailTool and MailTool Plus from Brad Stone.