Postman : Save requests and organize your APIs into Collections. Create and send any HTTP request using the awesome Postman request builder. Write your own test...
Webhook Tester : Webhook Tester allows you to easily test webhooks and other types of HTTP requests.
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
Mockbin : Mockbin allows you to generate custom endpoints to test, mock, and track HTTP requests & responses between libraries, sockets and APIs.
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