11.6.1 Username and Subaddress Substitution, $U, $0U, $1U

Any occurrences of $U in the template are replaced with the username (RFC 822 “local-part”) from the original address. Note that user names of the form a.“b” will be replaced by “a.b” as RFC2822 deprecates the former syntax from RFC 822 and it is expected that the latter usage will become mandatory in the future.

Any occurrences of $0U in the template are replaced with the username from the original address, minus any subaddress and subaddress indication character (+). Any occurrences of $1U in the template are replaced with the subaddress and subaddress indication character, if any, from the original address. So note that $0U and $1U are complementary pieces of the username, with $0U$1U being equivalent to a simple $U.