Rewrite username and subaddress substitutions

Rewrite username and subaddress substitutions, $U, $0U, $1U
Any occurrences of $U in the template are replaced with the username  (local part) from the original address. Note that usernames of the form a."b" will be replaced by "a.b" as current Internet standardization  work is deprecating the former syntax from RFC 822 and it is  expected  that the latter usage will become mandatory in future.

Any occurrences of $0U in the template are replaced with the username from the original address, minus any  subaddress (and  subaddress indication character  such as  ). Any occurrences of $1U in the template are replaced with the subaddress and subaddress indication  character, if any, from the original address. (See the discussion of the   channel option  and  Subaddresses in aliases  for background on subaddresses.) So note that $0U and $1U  are complementary pieces of the username, with $0U$1U being equivalent  to a simple $U.

$0U and $1U might be used when it is desired to force the account portion of the local-part to lowercase, while retaining original case in the subaddress since the subaddress indicates a folder name. For instance, a rewrite rule: org.domain.com  $\$0U$_$1U@org.domain.com will cause an address such as nAmE@org.domain.com to be transformed (rewritten) to name@org.domain.com, while an address such as  nAmE+sUbAdDrEsS@org.domain.com would be transformed to  name+sUbAdDrEsS@org.domain.com.

See also:
 * Rewrite rule templates
 * Subaddresses in aliases
 * subaddressexact Option
 * subaddress_char MTA Option
 * Rewrite rule template substitutions and control sequences