Ich würde auch immer mind. 2 aufsetzen, auch wenn es nur wenige User sind.
Ansonten den Tipp aus der Admin Hilfe befolgen:
If there are a small number of users on a server, the default (1) is usually sufficient. For larger numbers of users, set the number to 2 or higher. To determine the optimum number, enter SHOW STAT MAIL at the server console. If MAIL.WaitingRecipients is large or increasing, adding a mailbox may improve performance if the server resources are not overloaded.
You set the number of mail.boxes in the Configuration Settings document on the Router/SMTP - Restrictions and Controls - Basics tab. Under Router/SMTP Basics, enter a value for "Number of mailboxes."
Andreas