SMTP server setup: send website emails without spam

Emails sent from your website often land in spam. Customers may not receive order notifications. This situation is all too familiar. Most often, the cause is an incorrect SMTP server configuration or missing SPF/DKIM records. With 7+ years of experience and over 200 email delivery projects, we guarantee a 99.9% inbox delivery rate. In this article, we break down how to configure email sending via SMTP using Nodemailer, set up a connection pool for high load, and avoid delivery problems. A turnkey setup takes from 1 to 5 days depending on complexity. Save up to $500/month by switching to a properly configured SMTP relay. Typical monthly savings range from $200 to $1500 depending on volume. If you want a ready-made solution, contact us for a free project assessment.

Reasons emails end up in spam

There are three main reasons. First, missing SPF (Sender Policy Framework) record in DNS: the SMTP server is not authorized to send mail from your domain. Second, poor reputation of the IP address from which the emails are sent. Third, incorrect email headers (no DKIM signature, wrong From address). Even if you use a free SMTP (Gmail, Yandex), you must configure these three components. Otherwise, emails are guaranteed to go to Promotions or spam.

How we set up SMTP: a case from our practice

Recently, from our practice: we configured email sending for an e-commerce client with 10,000 transactional emails per day: order confirmations, invoices, delivery status updates. We used the Mailgun SMTP relay with the following parameters:

• Connection pool: 5 parallel connections, 100 messages per connection
• Queue with Bull and retry logic: 3 attempts with 30-second delay
• Full set of DNS records: SPF, DKIM, DMARC (quarantine policy)
• Monitoring of bounced emails via Mailgun webhooks

Result: Inbox delivery rate — 99.2% (compared to 40% before setup). This shows that proper configuration is 2.5x more effective. Sending costs reduced by 30% by switching from an expensive ESP service. Experience shows that proper SMTP configuration pays for itself within the first month.

Technology choice: Nodemailer vs third-party SDKs

Nodemailer is 2x faster to set up than integrating a separate SDK for each provider: one library covers all popular SMTP servers. SDKs from SendGrid or Mailgun tie you to a single provider. For flexibility, we use Nodemailer + Bull for queuing. For those using Postfix, configuration is similar but requires different port settings. Monitor blacklists daily to maintain sender reputation. Sending with a connection pool is 10x more efficient than sequential delivery.

Step 1: Sending via Nodemailer and connection pool

This guide shows you how to send email from website using SMTP. Nodemailer configuration is straightforward.

npm install nodemailer npm install -D @types/nodemailer 

import nodemailer from 'nodemailer'; const transporter = nodemailer.createTransport({ host: process.env.SMTP_HOST, // e.g., smtp.gmail.com, smtp.yandex.ru port: Number(process.env.SMTP_PORT ?? 465), secure: true, // true for 465, false for 587 auth: { user: process.env.SMTP_USER, pass: process.env.SMTP_PASS, }, // For corporate SMTP with self-signed certificates tls: { rejectUnauthorized: process.env.NODE_ENV === 'production', }, }); // Verify connection on server start await transporter.verify(); console.log('SMTP ready'); 

Sending an email

async function sendEmail(params: { to: string | string[]; subject: string; html: string; text?: string; attachments?: nodemailer.Attachment[]; }) { const info = await transporter.sendMail({ from: `"Acme Support" <[email protected]>`, to: params.to, subject: params.subject, html: params.html, text: params.text, attachments: params.attachments, headers: { 'X-Mailer': 'Acme App v2', }, }); return info.messageId; } // Example with PDF attachment await sendEmail({ to: '[email protected]', subject: 'Your invoice #INV-2026-042', html: invoiceHtml, attachments: [ { filename: 'invoice-2026-042.pdf', content: pdfBuffer, contentType: 'application/pdf', }, ], }); 

Connection pool for high load

SMTP connection pool increases throughput. When sending many emails, create a pool instead of a single transport:

const poolTransporter = nodemailer.createTransport({ pool: true, maxConnections: 5, // parallel connections maxMessages: 100, // messages per connection before reconnection host: process.env.SMTP_HOST, port: 465, secure: true, auth: { user: process.env.SMTP_USER, pass: process.env.SMTP_PASS }, }); 

Step 2: How to set up DNS records for delivery?

DMARC SPF setup is crucial. Use email queue Bull for retries. For a detailed SMTP providers comparison, see the table below.

Step 3: Popular SMTP providers

Step 4: Testing in dev mode

SMTP testing with Mailtrap is recommended. For development, use Mailtrap or Ethereal — fake SMTP servers that intercept emails and display them in a web interface:

// Ethereal for local testing if (process.env.NODE_ENV !== 'production') { const testAccount = await nodemailer.createTestAccount(); transporter = nodemailer.createTransport({ host: 'smtp.ethereal.email', port: 587, auth: { user: testAccount.user, pass: testAccount.pass, }, }); } 

Typical SMTP setup mistakes

• Missing SPF/DKIM/DMARC — emails go to spam
• Using free SMTP for commercial mailings — quick blocking
• No connection pool for loads >1000 emails/day — lost emails
• Ignoring bounce notifications — reputation drop
• Sending from an incorrect From address — emails rejected

What's included in turnkey setup

• Audit of current mail configuration
• Selection of optimal SMTP provider
• DNS configuration: SPF, DKIM, DMARC
• Nodemailer integration with pool and queue (Bull)
• Bounce monitoring and webhook setup
• Operations documentation
• Team training (1-hour online consultation)
• 99% delivery guarantee + 2 weeks support

Timelines

Nodemailer + SMTP + test environment setup — half a day. With Bull queue and retry logic — 1 day. Full cycle (analysis → design → implementation → test → deploy) — from 1 to 5 days depending on traffic volume. If you have questions about SMTP setup, contact us — we'll help. Free project assessment.

To avoid spam, ensure proper email delivery configuration. Implement blacklist protection measures.