You can use special variables in several places:
 - default_mail_env setting
 - namespace locations
 - static userdb template string
 - LDAP and SQL userdb query strings
 - log prefix for imap/pop3 process

The variables are:

 %u - username
 %n - user part in user@domain, same as %u if there's no domain
 %d - domain part in user@domain, empty if user there's no domain
 %h - home directory
 %p - protocol (IMAP or POP3)
 %P - PID of the current process (login or imap/pop3 process)
 %l - local IP address
 %r - remote IP address

You can apply a modifiers for each variable (eg. %Lp = pop3):

 %L - lowercase
 %U - uppercase
 %E - escape '"', "'" and '\' characters by inserting '\' before them.

You can take a substring of the variable by giving optional offset followed
by '.' and width after the '%' character. For example %2u gives first two
characters of the username. %2.1u gives third character of the username. If
offset points outside the value, empty string is returned.