Hosting Guide · Email
SMTP Relay Guide 2026: Brevo vs Mailgun vs Resend vs Amazon SES
If your website sends transactional emails — order confirmations, password resets, OTPs, invoice notifications — and those emails land in spam or fail to send at all, the problem almost certainly lies in your SMTP configuration.
Most hosting providers block outbound Port 25 to prevent spam abuse. This means PHP's mail() function and WordPress's wp_mail() cannot send email via localhost without a proper relay. The solution is an SMTP relay service — a dedicated third-party provider that handles email delivery on your behalf with high-reputation IP infrastructure.
This guide compares the four most popular SMTP relay services in 2026: Brevo, Mailgun, Resend, and Amazon SES — with verified pricing, feature breakdowns, and step-by-step WordPress setup instructions.
Contents
What Is an SMTP Relay Service?
An SMTP relay is a third-party mail server that accepts outgoing email from your application or website and delivers it to recipients through trusted infrastructure with an established IP reputation.
Rather than having your hosting server send mail directly (which is often blocked or flagged as spam due to poor IP reputation), you route email through a relay provider that maintains long-standing relationships with major inbox providers like Gmail, Outlook, and Yahoo — ensuring your messages land in the inbox, not the junk folder.
Email types that need SMTP relay
- Transactional email — Order confirmations, password resets, OTPs, invoices, receipts
- Notification email — System alerts, monitoring notifications, CRM messages
- Application email — Emails from CMS, eCommerce platforms, SaaS apps
- Bulk transactional — Sending invoices or statements to many recipients simultaneously
Note: SMTP relay for transactional mail is different from bulk email marketing (newsletters/campaigns), though some services like Brevo support both.
Why You Need an SMTP Relay
1. Port 25 Is Blocked by Most Hosting Providers
The standard SMTP port 25 is blocked on most shared and cloud hosting environments. When PHP calls mail() or WordPress calls wp_mail(), the function silently fails or the email never reaches the recipient. SMTP relay uses Port 587 (STARTTLS), which is never blocked by hosting providers.
2. Shared Hosting IPs Have Poor Reputation
On shared hosting, your IP address is shared with potentially hundreds of other websites. If any one of them sends spam, the shared IP ends up on a blacklist — and your legitimate transactional emails get rejected by Gmail or Outlook, even if you've done nothing wrong.
3. Email Authentication Is Complex to Self-Manage
Setting up SPF, DKIM, and DMARC correctly on a self-managed server is technically challenging. SMTP relay providers handle all of this automatically, including bounce processing, feedback loops, and unsubscribe handling.
4. Low Deliverability Without Proper Setup
Even if mail does technically send, without proper authentication it frequently ends up in spam. Poor deliverability means customers miss order confirmations, OTPs arrive late, and conversion rates drop.
Top SMTP Relay Services in 2026
All pricing data below was verified from official provider websites as of September 2026. Prices may change — verify current pricing at each provider's website before committing.
1. Brevo (formerly Sendinblue)
Brevo is one of the most widely used email platforms in Europe and Asia, popular among businesses that need both transactional email and marketing campaigns in a single platform. The interface is beginner-friendly and requires no technical background to get started.
- Free tier: Approximately 300 emails/day, unlimited contacts (verify at brevo.com)
- Starter plan: Priced by email volume, starting from 5,000 emails/month
- Strengths: Easy UI, built-in marketing automation, drag-and-drop editor, basic CRM, SMS campaigns
- Weaknesses: More expensive than Resend for pure transactional use; Brevo logo on free plan
- Best for: SMEs that want transactional email, newsletters, and marketing automation in one tool
* Brevo pricing changes frequently. Always verify at brevo.com/pricing
2. Mailgun
Mailgun (under Sinch Email group) is a battle-tested SMTP relay and email API service popular with developers. It offers powerful API access, detailed event logs, and a generous feature set — though its overage pricing is higher than Resend.
- Free tier: 100 emails/day (no monthly cap on days)
- Basic: $15/month for 10,000 emails/month; overage $1.80/1K
- Foundation: $35/month for 50,000 emails/month; overage $1.30/1K; 5-day log retention; 1,000 custom domains
- Scale: $90/month for 100,000 emails/month; dedicated IP pool; phone & chat support; 30-day log retention; SAML SSO
- Strengths: Rich API, email validation add-on, long log retention, advanced routing
- Weaknesses: High overage rates; UI feels dated compared to newer services
- Best for: Developer teams, SaaS apps, API-first workflows with detailed logging needs
Data as of September 2026 — verify at mailgun.com/pricing
3. Resend
Resend is a modern SMTP relay and email API that has quickly become the go-to choice for developers building with React, Next.js, and modern JavaScript frameworks. It features React Email template support, clean dashboard, and competitive pricing — especially for overage.
- Free: 3,000 emails/month (100/day cap), 3 domains, ticket support, 30-day retention
- Pro $20/month: 50,000 emails/month, 10 domains, unlimited daily sending, overage $0.90/1K
- Pro $35/month: 100,000 emails/month, overage $0.90/1K
- Scale $90–$1,150/month: 100K–2.5M emails/month; dedicated Slack channel; 1,000 domains; overage $0.46–0.90/1K
- Enterprise: Custom pricing for 3M+ emails; 99.99% uptime SLA; SSO; custom terms
- Strengths: Modern SDK (Node.js/Python/PHP/Go/Ruby), React Email integration, lowest overage rate, excellent DX
- Weaknesses: No built-in marketing email; newer company with shorter track record
- Best for: Developer-led teams, startups, Next.js/React applications
Data as of September 2026 — verify at resend.com/pricing
4. Amazon SES (Simple Email Service)
Amazon SES is the most cost-effective option for high-volume transactional email, especially for teams already using AWS infrastructure. It requires more technical setup than the others and starts in sandbox mode, but its per-email cost is unmatched at scale.
- Free tier: $200 AWS credits for new accounts (usable within 12 months)
- À la carte: $0.10/1,000 emails — no monthly subscription required
- Essentials plan: $0.16/1,000 emails (0–10M/month), decreasing at higher volumes
- Pro plan: $0.22/1,000 emails base, decreasing with volume
- Attachments: $0.12/GB of attachment data
- Strengths: Extremely cheap at high volume; native AWS integration; Lambda, SNS, CloudWatch
- Weaknesses: Sandbox mode requires production access request (1–2 business days); complex setup; minimal UI
- Best for: AWS-native teams, high-volume senders (>500K/month), technically strong teams
Data as of September 2026 — verify at aws.amazon.com/ses/pricing
Side-by-Side Comparison
| Service | Free Tier | Starting Plan | Cost/1K emails | Overage | Best for |
|---|---|---|---|---|---|
| Brevo | ~300/day | Volume-based | See site | See site | SME, all-in-one |
| Mailgun | 100/day | $15/mo (10K) | $1.30–1.80 | $1.30–1.80/1K | Developer, API |
| Resend | 3,000/mo | $20/mo (50K) | ~$0.40 | $0.90/1K | Dev, React/Next.js |
| Amazon SES | $200 credit | $0.10/1K | $0.10–0.16 | Per 1K | AWS, high volume |
* All prices verified from official pricing pages, September 2026. Verify current rates before purchasing.
How to Choose the Right SMTP Relay
By Monthly Email Volume
- Under 3,000/month: Resend Free or Brevo Free — no cost required
- 3,000–50,000/month: Resend Pro $20 (best value) or Mailgun Basic $15 for API features
- 50,000–500,000/month: Mailgun Scale, Resend Scale, or Amazon SES à la carte
- Over 500,000/month: Amazon SES is clearly the most cost-effective option
By Technical Requirements
- WordPress + simple plugin: All four services work (use SMTP username/password)
- Modern SDK & DX: Resend (SDK for Node.js, Python, PHP, Go, Ruby)
- API-first + detailed logs: Mailgun (event logs, email validation, advanced routing)
- Marketing automation too: Brevo (newsletter + transactional + CRM in one)
- AWS-native workloads: Amazon SES (Lambda triggers, SNS, CloudWatch alarms)
By Budget
For low-budget projects, Resend Free (3K/month) or Brevo Free (~300/day) covers most small websites at zero cost. When you need to scale, Resend Pro at $20/month offers the best value per email. For very high volumes, Amazon SES at $0.10/1K is the cheapest — but factor in the engineering time required for setup and maintenance.
Our Recommendation
For most websites and WordPress installs: start with Resend Free (3,000/month) or Brevo Free (~300/day) if you also need marketing. Upgrade to Resend Pro $20 as volume grows. For AWS-native enterprise workloads: Amazon SES is unbeatable on price.
WordPress SMTP Relay Setup: Step by Step
WordPress uses wp_mail() which relies on PHP's mail() by default — almost always blocked or unreliable. The fix: install an SMTP plugin that routes all outgoing email through your chosen relay.
Recommended WordPress SMTP Plugins
- WP Mail SMTP (by WPForms) — most popular, free and Pro tiers
- FluentSMTP — completely free, supports multiple providers, includes email log
- Post SMTP — free, email log, failure notifications
Setting Up Resend with WordPress
- Create a Resend account at resend.com and generate an API Key
- Add and verify your domain in the Resend dashboard (add the DNS records Resend provides)
- Install WP Mail SMTP or FluentSMTP in WordPress
- Choose "Other SMTP" and enter:
- SMTP Host:
smtp.resend.com - Port:
587(STARTTLS) or465(SSL) - Username:
resend - Password: your Resend API Key
- From Email: an email address at your verified domain
- SMTP Host:
- Send a test email and confirm it lands in the inbox
Setting Up Mailgun with WordPress
- SMTP Host:
smtp.mailgun.org - Port:
587(TLS) - Username: SMTP login from Mailgun dashboard (not your account email)
- Password: SMTP password from Mailgun (separate from your login password)
- Verify your domain in Mailgun first and add the provided DNS records
Setting Up Amazon SES with WordPress
- Create an AWS account and request production access (exit sandbox) via AWS Support
- Verify your domain in the SES Console and add the provided DNS records
- Create SMTP credentials in SES Console (separate from IAM credentials)
- Enter in your SMTP plugin:
- SMTP Host:
email-smtp.[your-region].amazonaws.com - Port:
587 - Username and Password from SES SMTP credentials
- SMTP Host:
Need Thai web hosting with full DNS management for SMTP relay setup?
Ruk-Com Cloud — Thai HostingDNS Records for Email Authentication
Every SMTP relay service requires DNS records for authentication before you can send email on behalf of your domain. There are three essential record types:
SPF Record (Sender Policy Framework)
Specifies which servers are allowed to send email from your domain. It's a TXT record on your root domain, for example:
v=spf1 include:spf.resend.com ~all
Each provider has its own include path. Never add multiple SPF records — combine them into one.
DKIM Record (DomainKeys Identified Mail)
A cryptographic signature that verifies the email hasn't been tampered with in transit. Each provider gives you a CNAME or TXT record to add to your DNS, for example: resend._domainkey.yourdomain.com
DMARC Record
Defines what happens to emails that fail SPF or DKIM checks. Start with monitoring mode:
v=DMARC1; p=none; rua=mailto:[email protected]
After a few weeks of clean reporting, upgrade to p=quarantine or p=reject for stronger protection.
Common Pitfalls to Avoid
Amazon SES Sandbox Mode
AWS SES starts all new accounts in sandbox mode, where you can only send to verified email addresses. You must submit a production access request via AWS Support — typically approved within 1–2 business days.
IP Warm-Up for Dedicated IPs
If you use a dedicated IP (available on higher-tier plans), you must warm it up by gradually increasing send volume. Sending too much too soon damages your IP's reputation. Most providers' shared IP pools are already warmed up and ready to use.
Keep Bounce Rate Below 5%
A bounce rate above 5% triggers account suspension with most providers. Regularly clean your email list, suppress bounced addresses immediately, and never purchase email lists.
Don't Use SMTP Relay for Bulk Marketing
SMTP relay services are designed for transactional email. Sending newsletters or promotional campaigns through them violates most providers' terms of service and can get your account suspended. Use Brevo (which explicitly supports both) or a dedicated email marketing platform.
From Address Must Match Verified Domain
All providers require your From address to use a domain you've verified with them. You cannot use a Gmail address or another provider's email as the From address.
Frequently Asked Questions
What is an SMTP relay service?
An SMTP relay is a third-party mail server that delivers email from your application to recipients through trusted infrastructure with established IP reputation, proper authentication (SPF/DKIM/DMARC), and bounce handling.
Why use SMTP relay instead of sending from hosting directly?
Most hosting providers block outbound Port 25 to prevent spam. Even if they don't, shared hosting IPs often have poor reputation, causing emails to land in spam. SMTP relay services use dedicated high-reputation IP pools with proper authentication configured.
Which SMTP relay has the best free tier?
Resend offers 3,000 emails/month (100/day) free with 3 domains — the most generous monthly allowance. Brevo offers around 300 emails/day with unlimited contacts. Mailgun offers 100 emails/day. For most small websites, Resend Free is the best starting point.
How do I set up SMTP relay in WordPress?
Install WP Mail SMTP or FluentSMTP, then configure it with the SMTP hostname, port 587, username, and password from your chosen relay provider. Each provider has WordPress-specific documentation. Send a test email to confirm delivery.
What DNS records do I need?
Three records: SPF (which servers can send from your domain), DKIM (cryptographic signature), and DMARC (policy for authentication failures). All four services in this guide provide step-by-step DNS setup wizards with automatic verification.
Which SMTP port should I use?
Port 587 (STARTTLS) is the modern standard and recommended by all providers. Port 465 (SSL/TLS) is also supported. Port 25 is blocked by virtually all hosting providers and ISPs — never use it.
Does Amazon SES require an AWS account?
Yes, you need an AWS account, must verify your domain or email address, and must request production access to exit sandbox mode (approved within 1–2 business days). In return, you get the cheapest per-email pricing available — $0.10/1,000 emails.
Can I use SMTP relay for newsletters?
Most SMTP relay services prohibit bulk marketing email. Brevo is the exception — it explicitly supports both transactional and marketing email. For newsletter-focused sending, use a dedicated email marketing platform.
Summary: Which SMTP Relay Should You Choose?
The right SMTP relay depends on your send volume, technical requirements, and budget:
- Starting out / testing: Resend Free (3K/month) or Brevo Free (~300/day)
- WordPress SME: Resend Pro $20/month or Brevo if you need marketing too
- Developer / SaaS team: Resend Pro or Mailgun Foundation
- High volume + AWS: Amazon SES at $0.10/1K — unbeatable at scale
Regardless of which service you choose, always configure SPF, DKIM, and DMARC correctly, verify your domain before sending, and keep your bounce rate under 5%. These three steps alone will dramatically improve your email deliverability.
Looking for Thai web hosting that supports full DNS management for SMTP relay setup? See AsiaGB or Ruk-Com Cloud — both offer DirectAdmin/cPanel with full DNS zone editors.