The best alternatives to Thunder Mail are Popcorn Notify, MailerSend, and Content Marketer. If these 3 options don't work for you, we've listed a few more alternatives below.
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Universal-2 by AssemblyAI— Speech-to-Text API for conversational data
MailerSend is a powerful transactional messaging service that gives developers a quick start in setting up transactional email and SMS, while also allowing marketing and design teams to easily contribute without coding skills.