Practical sieve examples seem kind of scarce, so...

example.sieve

# vim: ft=sieve

require "imap4flags";
require "fileinto";
require "copy";

if address :domain :is ["from", "to", "cc", "bcc"] "frobtech.com" {
  addflag "frobtech";

  if address :all :is "to" "report@frobtech.com" {
    addflag "\\seen";
    fileinto "Trash";
  }
}

if anyof (address :is "to" "meredith@quux.org",
          header  :is "list-id" "<quux.yahoogroups.com>",
          header  :is "list-id" "<quux.googlegroups.com>",
          address :is "reply-to" "webmaster@quux.org") {
  addflag "quux";
}

if header :is "list-id" "<jobs.perl.org>" {
  fileinto "Lists.perl-jobs";
}

if header :matches "x-original-to" ["root", "postmaster", "mailer-daemon*"] {
  fileinto "Alias.root";
}

if anyof (address :is "from" ["deliverysupport@safeway.com",
                              "service@chewy.com",
                              "billpayer@customercenter.net"],
          allof (address :is "from" "safeway@email.safeway.com",
                 header :matches "subject" "Reminder: Delivery * today")) {
  redirect :copy "xyzzy@gmail.com";
}

if allof (address :is "from" "leasing@landlord.com",
          header :is "subject" "Package Notification") {
  redirect :copy "foo@gmail.com";
}