Your emails aren’t reaching the inbox because of your infrastructure, not your message.
Behind every successful email campaign is something called an SMTP relay.
It’s the digital system that decides whether your messages hit the inbox or disappear completely.
As an email deliverability expert who has helped countless businesses launch and deliver successful email campaigns, let me show you:
- How does an SMTP relay work
- How do you choose the best one for you
- What are the best practices to boost your deliverability
Let’s dive in!
What is an SMTP relay?
An SMTP relay (Simple Mail Transfer Protocol relay) is how email servers pass messages from sender to recipient.
This happens especially when the two people use different domains or services. It’s the system that lets email cross from one place to another.
When you hit “Send” on an email, your message doesn’t instantly appear in someone’s inbox. It first goes to your outgoing mail server.
Then an SMTP relay hands it off to the recipient’s server, like a mail truck delivering a letter to the right address.
Without this relay process, your emails would never leave your domain. In business, that’s a complete roadblock to your campaign’s success.
Whether you’re sending personal emails or bulk marketing campaigns, SMTP relay is the infrastructure that moves your emails safely and reliably.
Get more emails to your customers’ inboxes with Maxify
Most SMTP relays just move your emails, but they don’t protect your reputation or maximize deliverability. That’s where Maxify Inbox is different.
We’ve built the only SMTP relay solution that combines infrastructure with intelligence:
- Build sender reputation gradually before scaling volume with automatic warmup
- Stop bad addresses from hurting your campaigns with email validation
- Use a dedicated IP so your deliverability isn’t affected by others
- Get expert guidance with unlimited deliverability consultations
Talk to our deliverability experts and discover how Maxify Inbox can transform your email performance.
Book your free consultation call.
How does SMTP relay work?
Every time you send an email, your message goes through a careful digital process. Here’s how it works step by step:
Step 1: Submission
Your email client (like Gmail or Outlook) sends the message to your SMTP server over a secure port. This is usually port 587 or 465. The relay begins here. Authentication often happens at this stage to prove you’re allowed to send emails from your domain.
Step 2: Domain resolution
The SMTP relay looks at the recipient’s domain (everything after the “@” symbol). It uses DNS MX records to figure out where to send the email.
Step 3: Connection and handshake
Your relay starts a connection with the recipient’s server using specific commands:
- HELO/EHLO (introduction)
- RCPT TO (who’s receiving)
- MAIL FROM (who’s sending)
This step makes sure both servers are ready to communicate.
Step 4: Security validation
The receiving server checks several things:
- DMARC checks if your policies match up
- SPF tells if this IP is allowed to send from your domain
- DKIM shows if the email was signed and not tampered with
TLS encryption also happens here, keeping everything private during transfer.
Step 5: Delivery or queueing
If everything looks good, the message gets delivered. If the server is busy or unreachable, the SMTP relay queues the email and tries again later.
What is the difference between SMTP relay and other email terms?
Understanding the differences between these components is important if you want to fix issues, improve deliverability, or scale your email setup.
1. SMTP
The protocol (rules) for sending email. It defines how email moves between servers. Used anytime an email is sent.
2. SMTP Relay
The process of transferring email from one server to another. Relays emails between domains or different servers. Used when sending emails outside your domain.
3. SMTP Server
The machine or software that sends emails. It starts or receives email transactions using SMTP. Used for sending internal or external emails.
4. Smart Host
A secure third-party SMTP relay that requires authentication. It provides an authenticated relay with better security and logging. It’s used for sending via external providers or enterprise-level routing.
5. SMTP Relay Service
A hosted solution (like Maxify) offering scalable email relays. It manages relaying, authentication, analytics, and deliverability, and is recommended to be used when sending mass, transactional, or automated emails.
Why does SMTP relay matter for getting your emails delivered?
If your email hits the spam folder, it doesn’t matter how good your message was.
Getting your emails delivered is about more than avoiding spammy phrases. It’s about the infrastructure that email providers trust.
SMTP relay sits right at the center of that trust, quietly deciding whether your email gets into the inbox or gets thrown away.
Here’s why your relay setup could be killing your email success:
Sender authentication builds trust
SMTP relays enable authentication protocols that prove you’re a legitimate sender. Without these authentication methods, your emails look suspicious even when they’re perfectly legitimate.
Bounce management protects your reputation
Email providers track bounce rates to assess your sending practices, and SMTP relays help minimize the damage from delivery failures.
Smart relays can distinguish between hard bounces (permanent failures) and soft bounces (temporary issues), ensuring you don’t waste time retrying impossible deliveries.
Retry logic ensures temporary failures don’t result in lost messages by attempting delivery at appropriate intervals.
Automatic suppression of invalid addresses prevents you from repeatedly sending to dead email addresses that hurt your reputation.
Reputation protection at scale
Shared IP addresses mean shared consequences, but dedicated relays keep your sending isolated and clean.
A dedicated SMTP relay and IP address protect you from bad neighbors whose poor practices could damage your reputation.
Properly warmed-up relays signal responsible sending behavior to email providers by gradually building trust over time.
Separating your marketing and transactional emails reduces risk across your entire email program since each type has different sending patterns and requirements.
Encryption secures the journey
Every hop between servers creates a potential security vulnerability unless you’re properly protected.
Encrypted relays also help you meet privacy regulations and compliance standards that govern how customer data must be handled.
Email provider compliance keeps you out of trouble
Even excellent content gets filtered if you send too quickly or too frequently for the provider’s comfort.
SMTP relays control your sending volume to stay within the limits set by major email providers like Gmail and Outlook.
Intelligent pacing algorithms help you avoid triggering spam filters that activate when they detect unusual sending patterns.
Many professional relay services maintain direct relationships with major email providers, which can significantly boost your inbox placement rates.
Which SMTP ports should you use?
Every SMTP relay uses specific port numbers to connect between servers. These ports are like doorways. Some are locked, some are monitored, and some are built for high-speed, secure passage.
Here’s what you need to know about common ports:
| Port | Name | Use case | Encryption | Commonly used for |
| 25 | Standard SMTP | Traditional SMTP used for server-to-server relays. Often blocked by ISPs. | None (or optional) | Internal relays, legacy systems |
| 465 | SSL/TLS Port | SMTP over SSL. Deprecated, but still supported by many providers. | SSL | Legacy systems needing encrypted connections |
| 587 | Submission Port | Official port for sending mail securely with authentication. | STARTTLS | Most common for email clients & authenticated relays |
| 2525 | Alternative Port | Used when 587 is blocked. Not officially recognized but widely supported. | STARTTLS or SSL | Backup option, especially for cloud relays |
Port 587 is the gold standard for authenticated, secure sending. It’s your best choice for most SMTP relay configurations.
Port 25 is often blocked to prevent spam abuse. If 587 doesn’t work, 2525 is your backup plan.
What are common SMTP relay problems and how do you fix them?
Even the best SMTP relay setup can run into trouble. Here’s how:
Blocked IP address
Your IP gets flagged as spam, and your emails disappear into filters.
The best approach is to warm up your IP before bulk sending by gradually increasing your email volume over time.
If you’re already blocked, request removal from blocklists with a clear explanation of what you fixed and how you’ll prevent future issues.
Using a dedicated IP gives you complete control over your reputation since you won’t be affected by other senders’ bad practices.
High bounce rates
Too many undelivered emails create an instant red flag with email providers.
You need to validate your email list before every campaign to catch invalid addresses before they cause problems.
Remove hard bounces immediately since these addresses will never work.
Keep your lists organized to maintain high engagement rates, and monitor SMTP responses to understand whether bounces are caused by full inboxes or truly invalid addresses.
Delayed deliveries
Your emails take hours or days to arrive, which can kill time-sensitive campaigns.
Start by checking your SMTP server load since an overloaded server will create delays. Optimize your message size by avoiding large attachments that slow down processing.
Also, make sure your retry intervals and timeout settings are configured correctly for your sending volume.
Consider using a relay service with queue management and smart routing to handle delays automatically.
Authentication failures
Servers reject your emails before they even reach the inbox.
Make sure your SPF, DKIM, and DMARC records are set up correctly since these are the foundation of email authentication.
Confirm that SMTP AUTH is enabled on your relay to prove you’re authorized to send.
Check that your domain alignment is correct across all email headers since mismatched domains trigger spam filters.
Wrong SMTP settings
Even small mistakes in your configuration can block everything from working.
Double-check your SMTP server address, port number, and security settings since these are common sources of errors.
Test your sending using tools to verify that your connection works properly.
Review your server logs to trace exactly where failures happen so you can fix the root cause.
How do you choose the right SMTP relay provider?
The wrong provider won’t just slow your emails but also hurt your reputation. Here’s how to choose one that won’t leave your emails in limbo:
Reputation management
See if it offers:
- Real-time blocklist and bounce rate monitoring
- Automatic warmup processes to build sender trust
- Support for SPF, DKIM, and DMARC authentication setup
- Dedicated IPs rather than shared pools, where bad neighbors hurt your reputation
Analytics and insights
Check if you’re getting:
- Performance-based sending optimization tools
- Real-time reputation trend dashboards with early warning alerts
- Provider-specific feedback (Gmail vs Outlook filtering differences)
- Comprehensive tracking for opens, clicks, bounces, and deliveries
Easy integration
Can you integrate easily? Check if you have:
- Seamless integration with CRMs, content management systems, and automation tools
- Pre-built plugins for WordPress, Shopify, HubSpot, and other platforms
- Webhook support for automated bounce and unsubscribe handling
- Robust API access for email process automation
Reliability and support
Can you rely on it? Check if you have:
- Deliverability consultations beyond basic technical support
- 99.9% uptime guarantees with transparent status pages
- Dedicated onboarding assistance for faster setup
- 24/7 live support (not just email tickets)
Fair pricing
Money matters, especially in the long run. Find out if you get:
- Reasonable scaling that grows with your business
- Transparent limits on volume, sending speed, and IP addresses
- All-inclusive pricing (no hidden charges for analytics or support)
- Flexible trial periods or monthly plans before long-term commitments
What are advanced SMTP relay features?
Most email marketers only use basic SMTP relay features.
But for enterprises, developers, and high-volume senders, advanced configuration makes the difference between the inbox and the spam folder.
Here’s how modern SMTP relays work for performance, protection, and scale:
Authentication and security
Relay abuse is real, and authentication is your first line of defense:
- Modern relays require SMTP AUTH to verify sender identity
- Advanced relays enforce security policies at the connection level
- Encryption via SSL/TLS (on ports 465/587) protects credentials and content
Smart routing and delivery
SMTP relays don’t just have to deliver, but deliver intelligently:
- Enterprise systems use load balancing and retry logic for maximum throughput
- Relays prioritize based on MX weightings for backup options and speed
- Use MX record lookups to find optimal delivery paths
Bulk email at scale
If you’re sending at scale, your SMTP relay better be built for it:
- Handles mass delivery without slowing your main server
- Tracks bounce patterns, open rates, and delivery metrics in real-time
- Manages email queues and enforces rate limiting to follow provider rules
Preventing abuse
Open relays are outdated and dangerous:
- Modern relays are closed by default, only allowing approved or authenticated senders
- Built-in alerts flag suspicious patterns before email providers do
- Use IP reputation checks, spam scoring, and usage limits
Modern integrations
SMTP relays today integrate seamlessly:
- APIs, SDKs, and webhooks enable real-time automation
- Integrates seamlessly with CRMs, web apps, and marketing platforms
- Monitoring tools help developers debug, test, and optimize infrastructure
- Supports transactional templates, variable substitution, and dynamic content
Why choose Maxify Inbox for SMTP relay?
Maxify Inbox combines enterprise-grade SMTP infrastructure with automated reputation protection. While other providers just move emails, we ensure they actually get delivered.
Here’s what makes us different:
- Automatic IP warmup that builds trust before you scale
- Real-time email validation that protects your reputation
- Dedicated IPs with no risk from bad neighbors
- Unlimited deliverability consultations with experts
- Complete authentication setup (SPF, DKIM, DMARC)
Ready to stop losing emails to spam folders? Book your free consultation today.
Frequently asked questions
Here are some commonly asked questions about SMTP relay:
What is the SMTP relay?
An SMTP relay is the process of transferring an email from your sending server to the recipient’s server. It allows messages to travel across different domains and email platforms. This is essential for delivering emails outside your internal network.
What is SMTP used for?
SMTP, or Simple Mail Transfer Protocol, is the standard protocol used to send emails. It defines the rules and steps servers follow to transmit messages reliably and securely over the Internet.
What is the SMTP relay for email routing?
SMTP relay handles the routing of emails between different domains. When you send an email outside your organization, the relay ensures it reaches the correct destination by looking up the recipient’s server through DNS.
What is the SMTP relay handshake?
The SMTP handshake is the initial exchange between sending and receiving servers. It includes commands like HELO, MAIL FROM, and RCPT TO, ensuring both servers are ready to transfer and accept the email securely.
