An example of how to resend messages using the Mailgun API. Per the documentation at https://documentation.mailgun.com/en/latest/api-sending.html#examples. Find your Mailgun Private API Key: https://app.mailgun.com/app/account/security/api_keys

resend.rb

mailgun_private_api_key = "key-TKTK"

urls = %w[
https://se.api.mailgun.net/v3/domains/{{YOUR_DOMAIN_NAME}}/messages/{{MESSAGE_STORAGE_KEY}}?to=foo@bar.com
https://storage-us-west1.api.mailgun.net/v3/domains/{{YOUR_DOMAIN_NAME}}/messages/{{MESSAGE_STORAGE_KEY}}?to=foo@bar.com
]

urls.each do |url|
  uri = URI.parse(url)
  params = Hash[URI.decode_www_form(uri.query)]
  http = Net::HTTP.new(uri.host, uri.port)
  http.use_ssl = true
  http.verify_mode = OpenSSL::SSL::VERIFY_NONE
  request = Net::HTTP::Post.new(uri.request_uri)
  request.basic_auth("api", mailgun_private_api_key)
  request.add_field('Content-Type', 'application/json')
  request.body = params.to_json
  response = http.request(request)
  puts "#{JSON.parse(response.body).dig('message')} // #{url}"
end