By default, Virtualmin will append or prepend either the username or the first part of the domain name to all mailbox users created in this domain, in order to ensure there are no username clashes. However, this field can be used to specify a different prefix or suffix, which can be useful if your system has a short username length like (like FreeBSD) and the default prefix is too long.

How the prefix is selected by default is determined by the Domain name style in username option on the Module Config page. If it set to Username or first part of domain, the prefix will be the same as the domain's user and group name. If set to Full domain name, it will always be the virtual server's domain name. Otherwise, it will be the first part of the domain name (unless there is a clash with another domain's prefix).