Update README.md (#827)

README.md

 * [Fetch](https://github.com/tedious/Fetch) - An IMAP library.
 * [Mautic](https://github.com/mautic/mautic) - Email marketing automation
 * [PHPMailer](https://github.com/PHPMailer/PHPMailer) - Another mailer solution.
-* [Stampie](https://github.com/henrikbjorn/Stampie) - A library for email services such as [SendGrid](https://sendgrid.com/), [PostMark](https://postmarkapp.com), [MailGun](https://www.mailgun.com/) and [Mandrill](http://www.mandrill.com).
+* [Stampie](https://github.com/Stampie/Stampie) - A library for email services such as [SendGrid](https://sendgrid.com/), [PostMark](https://postmarkapp.com), [MailGun](https://www.mailgun.com/) and [Mandrill](http://www.mandrill.com).
 * [SwiftMailer](http://swiftmailer.org/) - A mailer solution.
 
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