You can use special variables in several places:
 - mail_location 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, POP3, smtp, deliver (unfortunately mixed casing is used))
 %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. %Ls = pop3):

 %L - lowercase
 %U - uppercase
 %E - escape '"', "'" and '\' characters by inserting '\' before them.
      Note that variables in SQL queries are automatically escaped, you
      don't need to use this modifier for 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.
 %M - return the string's MD5 sum
 %D - return "sub.domain.org" as "sub,dc=domain,dc=org" (for LDAP queries)

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 the offset is negative, it counts from the end, for example %-2.2i gives
the UID mod 100 (last two characters of the UID printed in a string). If a
positive offset points outside the value, empty string is returned, if a
negative offset does then the string is taken from the start.

If the width is prefixed with zero, the string isn't truncated, but only
padded with '0' character if the string is shorter. For example %04i may
return "0001", "1000" and "12345". %1.04i for the same string would return
"001", "000" and "2345".

For login_log_format_elements there are also these variables:

 %m - authentication method (eg. PLAIN)
 %a - Local port
 %b - Remote port
 %c - SSL, TLS or empty
