<i>Do all users need their own directory entry to send emails.</i>
No. They only need a directory entry if they’re using SNADS functions. If the e-mail is going to be all SMTP, then there probably shouldn’t be any SNADS elements dragged into it.
<i>Is there a way for anyone to signon to the system and use a CL program that sends an email message by SNDDST.</i>
Yes. First, signon to the system with a valid profile and password. Then, call a CL program that includes SNDDST.
I mean, what other answer is possible?
A little beyond that, if you use SNDDST, then you’re dragging SNA functions into the situation. If all you want is SMTP e-mail, why would you want to use a SNA-based command? That’s the first element that causes the system directory entry to be needed. (Your question has “SMTP” in the title rather than “SNA”.)
Note that you <i>might</i> be able to get by with a single directory entry if you have a generic user ID and reply address that everyone may use. The SNDDST can be executed after switching to the generic user as the current-user and switching back after SNDDST runs.