1) Login to your SmarterMail account as 'admin' user.
2) Click on Settings-> Expand the domain settings->Expand the Mailing List.
3) Click on Mailing List->New
4) Enter the list name (example[at]domainname.com).
5) Choose the moderator, i.e. the user which can use this mailing list, Set the password and Subject Prefix.
6) Click on Save button.
7) Now click on subscriber->New.
8) Enter the recipient mail address one per line in the box given.
9) Click on ok.

Now we can send the mail to all the subscribers at once using the single mailing list name example[at]domainname.com