In Lookafter system, postmaster is a reserved catch-all user account that will capture and store all emails sending to non-existent users in your domain.

For example, Amy sends an email to David at david7@mydomain.com, but David's actual email address is david1@mydomain.com.
In usual case, such email will be bounced, and Amy has to resend the email to David's correct email address.
If postmaster@mydomain.com is available, then the email will be received in postmaster@mydomain.com mailbox.

Note that the postmaster user account is not created in your Virtual Office by default.
If it is needed, you may request your Avomaster/voadmin to create the user account (with “postmaster” as the username).