Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Waitlist API - Quick and easy waitlist with built in referral.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
LaunchList - Create waitlist with referral for your product launch
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
Waitlisty.io - Build excitement for your product by embedding a simple form into your website. Signups will receive a referral code to share with their friends. The more codes they share, the more they climb your waitlist.