Is this a genuine question? I dont want to be rude but I dont understand how are you asking this?

Just in case it is a genuine question:

- This is a tool for designing/creating emails - they say in the title of this post "Open source HTML email designer"

- Resend is a tool for sending emails: "Deliver transactional and marketing emails at scale" this is from the hero of resend homepage

I dont know any of them, just opened the Resend website and read the hero and read the description of the post you are commenting.

Do your research before responding to me next time. They're both email design tools - Resend is based around building your emails in React.