Additional email addresses or aliases are added through the SmarterMail web interface after logging in as the Primary Admin, which defaults to email@example.com. If the postmaster password is unavailable, log in automatically at Winhost Control Panel > Sites > the applicable domain name > Email > "SmarterMail Manager" link.
After logging in as the Primary Admin, click Domain Settings1 (gear with small globe) then Accounts2. If the menu on the left with "Accounts" is not being displayed, click the hamburger icon3 in the upper left of the screen to expand the menu.
From the Accounts screen, click "Users" or "Aliases" in the right pane then "New" to create the new email address. When creating a new email user, note that the "username" is the new email address whereas for an email alias it is the "name".
Users > New > Username "new" creates the email address firstname.lastname@example.org
Alias > New > Name "new" creates the email alias email@example.com