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 login/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
 %s - service (IMAP or POP3)
 %p - PID of the current process (login or imap/pop3 process)
 %l - local IP address
 %r - remote IP address
 %w - plaintext password from plaintext authentication mechanism
 %i - System UID of the user

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

 %L - lowercase
 %U - uppercase
 %E - escape '"', "'" and '\' characters by inserting '\' before them.
 %R - reverse the string
 %H - take a 32bit hash of the variable and return it as hex. You can also
      limit the hash value. For example %256Hu gives values 0..ff. You might
      want padding also, so %2.256Hu gives 00..ff. This can be useful for
      example in dividing users automatically to multiple partitions. Note
      that if you're hashing usernames being in user@domain form, you probably
      want to reverse the string to get better hash value variety, eg. %3RHu.

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.

For login_log_format_elements there are also these variables:

 %m - authentication method (eg. PLAIN)
 %c - SSL, TLS or empty
