If the issue is with Resend, on the Resend dashboard does it show yellow or red colors? If everything is all green on Resend, then the issue may be in your implementation. Make sure you included your .env keys on Vercel for Resend. Sometimes that can be forgotten. Also make sure your .env keys are set to your actual website and not local host. Since Resend is a Serverless Function, you can also check your log on the Vercel dashboard for errors. You can easily test this in prod by going to the location where an email should be sent, clicking the button, and quickly checking your server log on Vercel. This should tell you what the error is.