Derpy is a reddit mail notifier. She alerts you to new messages!
Derpy Orangereds combines reddit mail checking and ponies together. Periodically, Derpy will check to see if you have new mail, and if so, the mail icon will change and you'll hear a little sound alert (you can turn off the sound if you want).
Additionally, you can replace Derpy with the Scootacourier service, which will reskin the icons and change the sound to Scootaloo saying "Telegram!"
Or if you love Sweetie Belle, you can switch to the magical Sweetie Belle MailHype(tm) service, to hear the young unicorn gleefully tell you about new messages!