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insomnia
The open-source, cross-platform API client for GraphQL, REST, WebSockets, SSE and gRPC. With Cloud, Local and Git storage.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Sure. I have an account with mailgun, though I have heard good things about sendgrid as well. It costs me $35/mo, but I use the account as part of the management of a phpbb forum I host, but I have a patreon for that that covers the expenses. If I didn't have that patreon, I would have to think pretty hard about how to proceed.
Mailgun has a nice web UI for managing routes, but it's much faster to just fire off an API call. For this I use insomnia on my personal machines, running linux(but insomnia does work on windows too, if you use that), paw when I occasionally use my work mac for route management(which is not often), and restler for android.