The Sieve Mail Filtering Language

Here is a Sieve filter I use on Protonmail (free account), which shows some of the things that are possible. The labels it assigns have to be created manually first:

    require ["vnd.proton.expire", "fileinto", "variables", "imap4flags", "relational"];
    ## Detect email forwarded from Gmail and set them to automatically delete after 60 days
    if anyof(header :matches "Return-Path" "*+caf_*@gmail.com*", header :matches "X-Forwarded-For" "*@gmail.com*"){
      expire "day" "60"; # set expiry to 60 days (uses custom Protonmail extension)
      fileinto "Gmail"; # file into Gmail folder
    }
    ## Add name field as label
    ## 1st * matches sender name as ${2}
    ## This doesn't work because automatic label/folder creation is not supported by Protonmail
    #if header :matches "from" "?*? <*" {
      # fileinto "Gmail/${2}"; 
      # fileinto :create "Gmail/${2}";
    #}

    ## Try to detect newsletters based on header
    if exists "list-unsubscribe" {
      expire "day" "14"; # set expiry to 14 days
      fileinto "Newsletter"; # Assign "Newsletter" label
    }
    ## Add label based on size
    if size :over 8M{
      fileinto "8M+";
    } elsif size :over 4M{
      fileinto "4M+";
    } elsif size :over 2M{
      fileinto "2M+";
    } elsif size :over 1M{
      fileinto "1M+";
    } elsif size :over 512K{
      fileinto "512K+";
    } elsif size :over 256K{
      fileinto "256K+";
    } elsif size :over 128K{
      fileinto "128K+";
    }
    ## Add label based on spam score (score of 0 means no score assigned)
    if header :matches "X-Pm-Spamscore" "*" { #detect Protonmail spam score in header
      if string :value "ge" "${1}" "1"  { #if spam score >= 1
        fileinto "spam score ${1}"; # assign label with spam score
      }
    }