Fastmail sieve - put email aliases into their own subfolder

fastmail.sieve

#
# My custom sieve script for putting emails into their own subfolder
# Ref: https://www.reddit.com/r/fastmail/comments/qahdf3/my_custom_sieve_script_for_putting_emails_into/
#
if exists "X-Delivered-to" {
  # An email addressed to 
  #   "website@user.domain.com" will be put in the folder "Website"
  #   "my-website@user.domain.com" will be put in the folder "My Website"
  if header :regex "X-Delivered-to" "([A-z0-9]+)-?([A-z0-9]+)?@user.domain.com" {

    if string :is "${2}" "" {
      # Only first word is being used
      set :lower :upperfirst "website" "${1}";
    } else {
      # Two words are being used, let's capitalize both of them
      set :lower :upperfirst "one" "${1}";
      set :lower :upperfirst "two" "${2}";
      set "website" "${one} ${two}";
    }
    # Move email into its own folder under the inbox
    fileinto :create "INBOX.${website}";
  }
}

See also:

• https://www.fastmail.com/developer/integrating-with-fastmail/
• https://github.com/TomasHubelbauer/fastmail
• https://www.ietf.org/id/draft-ietf-jmap-sieve-13.html#name-sievescript-set
• https://dague.net/2017/12/13/syncing-sieve-rules-in-fastmail-the-hard-way/