Litmus : Test and track your emails. Email previews, analytics and spam filter testing.
mail-tester.com : Free online service that allows you to test your emails for Spam, Malformed Content and Mail Server Configuration problems
Webhook Tester : Webhook Tester allows you to easily test webhooks and other types of HTTP requests.
Postal : Postal is a complete and fully featured mail server for use by websites & web servers. Think Sendgrid, Mailgun or Postmark but open source and ready f...
httpbin.org : A simple HTTP Request & Response Service.
MailHog is an email testing tool for developers:
Configure your application to use MailHog for SMTP delivery
View messages in the web UI, or retrieve them with the JSON API
Optionally release messages to real SMTP servers for delivery