Cursor
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SMTP Connection Refused in Production for Cursor-Generated Email Code
Your Cursor-generated application sends emails perfectly during local development but fails with SMTP connection errors in production. Users aren't receiving signup confirmations, password resets, or notification emails. The server logs show connection refused, timeout, or authentication errors when attempting to connect to the mail server.
Email delivery is critical infrastructure — when it breaks, users can't sign up, can't reset passwords, and lose trust in your application. Cursor often generates SMTP code configured for a local mail server or development service like Mailtrap/Mailhog, which doesn't translate to production.
The issue might also manifest as emails being sent successfully according to your application logs, but never arriving in users' inboxes due to DNS, SPF, or deliverability configuration problems that only affect production domains.
Error Messages You Might See
Error: connect ECONNREFUSED 127.0.0.1:25
Error: connect ETIMEDOUT smtp.gmail.com:587
Error: Invalid login: 535 5.7.8 Authentication failed
Error: self signed certificate in certificate chain
getaddrinfo ENOTFOUND smtp.mailtrap.io
Common Causes
- Port 25 blocked by cloud provider — AWS, GCP, Azure, and most cloud providers block outbound SMTP port 25 by default to prevent spam. You need to use port 587 (STARTTLS) or 465 (SSL/TLS)
- Development SMTP credentials in production — Cursor configured Mailtrap, Mailhog, or localhost:1025 credentials that only work in development
- Missing or wrong TLS/SSL configuration — Production SMTP servers require TLS encryption but the code uses plain text connections, or uses SSL when STARTTLS is expected
- Firewall or security group blocking outbound — The production server's security group or network ACL doesn't allow outbound connections on port 587 or 465
- SMTP authentication method mismatch — The email provider requires OAuth2 authentication (like Gmail) but the code uses plain username/password
- DNS resolution failing for SMTP host — The SMTP hostname can't be resolved from the production environment due to internal DNS or networking constraints
How to Fix It
- Use port 587 with STARTTLS — Configure your SMTP connection to use port 587 with STARTTLS encryption. This is the standard for authenticated email submission and is not blocked by cloud providers
- Switch to an email API service — Replace SMTP with an HTTP-based email API like SendGrid, Resend, Postmark, or AWS SES. These are more reliable than SMTP in serverless/cloud environments and include deliverability features
- Verify production environment variables — Check that SMTP_HOST, SMTP_PORT, SMTP_USER, and SMTP_PASS are set correctly in your production environment, not pointing to development values
- Check cloud provider email restrictions — If on AWS, you may need to request removal of the port 25 throttle. On most providers, using SES or a third-party email API is the recommended approach
- Test SMTP connectivity from production — SSH into your production server and run
openssl s_client -connect smtp.provider.com:587 -starttls smtpto verify the connection is possible - Configure SPF, DKIM, and DMARC — Even if SMTP connects, emails may be rejected without proper DNS records for your sending domain
Real developers can help you.
Matthew Jordan
I've been working at a large software company named Kainos for 2 years, and mainly specialise in Platform Engineering. I regularly enjoy working on software products outside of work, and I'm a huge fan of game development using Unity. I personally enjoy Python & C# in my spare time, but I also specialise in multiple different platform-related technologies from my day job.
Tejas Chokhawala
Full-stack engineer with 5 years experience building production web apps using React, Next.js and TypeScript.
Focused on performance, clean architecture and shipping fast.
Experienced with Supabase/Postgres backends, Stripe billing, and building AI-assisted developer tools.
Matthew Butler
Systems Development Engineer @ Amazon Web Services
Stanislav Prigodich
15+ years building iOS and web apps at startups and enterprise companies. I want to use that experience to help builders ship real products - when something breaks, I'm here to fix it.
Vlad Temian
15+ years shipping production infrastructure for startups. Former CTO at qed.builders (acquired by The Sandbox).
Cursor ambassador and agentic tooling builder.
I've scaled systems, automated deployments, and built observability tools for AI coding workflows. I specialize in taking vibe-coded apps from broken prototype to production-ready: fixing Supabase auth/RLS, Stripe integrations, deployment pipelines, and cleaning up AI-generated spaghetti.
I build tools in this space (agentprobe, claudebin, micode) and understand both sides: how AI generates code and why it breaks.
https://blog.vtemian.com/
Taufan
I’m a product-focused engineer and tech leader who builds scalable systems and turns ideas into production-ready platforms.
Over the past years, I’ve worked across startups and fast-moving teams, leading backend architecture, improving system reliability, and shipping products used by thousands of users. My strength is not just writing code — but connecting product vision, technical execution, and business impact.
Dor Yaloz
SW engineer with 6+ years of experience,
I worked with React/Node/Python
did projects with React+Capacitor.js for ios
Supabase expert
Antriksh Narang
5 years+ Experienced Dev (Specially in Web Development), can help in python, javascript, react, next.js and full stack web dev technologies.
Mehdi Ben Haddou
- Founder of Chessigma (1M+ users) & many small projects
- ex Founding Engineer @Uplane (YC F25)
- ex Software Engineer @Amazon and @Booking.com
Caio Rodrigues
I'm a full-stack developer focused on building practical and scalable web applications. My main experience is with **React, TypeScript, and modern frontend architectures**, where I prioritize clean code, component reusability, and maintainable project structures.
I have strong experience working with **dynamic forms, state management (Redux / React Hook Form), and complex data-driven interfaces**. I enjoy solving real-world problems by turning ideas into reliable software that companies can actually use in their daily operations.
Beyond coding, I care about **software quality and architecture**, following best practices for componentization, code organization, and performance optimization.
I'm also comfortable working across the stack when needed, integrating APIs, handling business logic, and helping transform prototypes into production-ready systems.
My goal is always to deliver solutions that are **simple, efficient, and genuinely useful for the people using them.**
You don't need to be technical. Just describe what's wrong and a verified developer will handle the rest.
Get HelpFrequently Asked Questions
Why does email work locally but not in production?
Local development often uses a local SMTP server (Mailhog, Mailtrap) or your machine has no firewall restrictions. Production environments block port 25, require TLS, and need real SMTP credentials. Switch to an email API service like SendGrid or Resend for reliable production email.
Should I use SMTP or an email API?
For production apps, use an HTTP-based email API (SendGrid, Resend, Postmark, AWS SES). They're more reliable than SMTP in cloud/serverless environments, handle retries automatically, and include deliverability monitoring and analytics.