Resend's DNS records (SPF, DKIM, etc.) need to be added at your domain's DNS provider, and neither Railway-provided domains (*.up.railway.app) nor Railway-purchased domains support adding custom DNS records. You'll need a domain from an external registrar (like Cloudflare, Namecheap, etc.) where you have full DNS control to add Resend's verification records, and you can then use that same domain as a custom domain on your service.

The bot is correct.