Rails HTML Email to Text

application_mailer.rb

class ApplicationMailer < ActionMailer::Base
  using Refinements::Mailings

  protected

  def render_text_for(template_name)
    text = render_to_string(template_name, formats: :html).html_to_plain
    render(plain: text)
  end
end

my_mailer.rb

class MyMailer < ApplicationMailer
  def welcome_email
    mail(subject: "Welcome") do |format|
      format.html
      format.text { render_text_for(__method__) }
    end
  end
end

refinements_mailings.rb

module Refinements::Mailings
  refine String do
    def html_to_plain
      preprocessed = self
        .gsub("<hr>", "\n--- --- --- --- --- --- --- --- --- --- --- ---\n")
        .gsub(/<br\\?>/, " ") # line breaks
        .gsub(/<li[^>]*>/, "- ") # lists
        .gsub(/<\/t[hd]>\s*<t[hd][^>]*>/, " | ") # table cells (tr and table tags are removed later)

      return ActionController::Base.helpers.strip_tags(preprocessed)
        .split("\n").map(&:strip).join("\n") # fix indentation
        .gsub("\n\n\n", "\n") # remove extensive new lines
        .strip
    end
  end
end

Source: https://www.netskin.com/posts/20230716-autogenerate-plain-email-version