Mailtrack.io - The best email tracking solution I've found. Free
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Campaign Monitor - Email marketing software built for designers and their clients to run successful email campaigns.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
Inbox Grader by Freshworks - Free metrics 📈 for your business and team email ✉️
Plivo - Plivo simplifies your customer engagement.