SMTP vs. API? I just want to send my email!
If that’s you, don’t worry. Sending email comes with a lot of jargon, but you don’t need to know all of it to make the right call.
When you use SendLayer as your email provider, you get two ways to connect your website or app: SMTP and API. Both have pros and cons.
In this post, I’ll explain the difference in plain English, so you can pick the right one for your needs.
SMTP vs. API for Sending Emails
SendLayer gives you two ways to send transactional email: over SMTP or with the SendLayer API.
SMTP is the traditional method. It’s a protocol that has been the standard for sending email since the 1980s. It works by relaying your message from server to server until it arrives.
An email API works differently. It sends email straight from your app to your email provider, with no separate mail server in between.
Sometimes the choice is made for you. Many websites let you set custom SMTP details but offer no API option at all.
But if you’re building an app, or a plugin or site lets you pick, it helps to know the difference. Your choice affects deliverability, speed, scale, and how well you can track performance.
SMTP vs. API at a Glance
Here’s how the two methods compare at a high level. The details follow below.
| Factor | SMTP | Email API |
|---|---|---|
| How it connects | Relays through a mail server | Sends over HTTP straight to the provider |
| Setup | Simple, minimal config | Needs some developer work |
| Speed at volume | Slower (handshake + retries) | Faster, built for bulk |
| Analytics | Basic delivery only | Opens, clicks, bounces, engagement |
| Scalability | Fine for steady volume | Scales easily with traffic |
| Attachments (SendLayer) | Up to 10 MB | Up to 25 MB |
| Firewall/ports | Can be blocked on some networks | Uses HTTP, rarely blocked |
| Best for | Most sites, plugins, and casual senders | High volume, apps, detailed tracking |
Neither one is “better.” The right pick depends on how you send and how much. We’ll break down each method next.
What Is SMTP?
Simple Mail Transfer Protocol (SMTP) is a way to send email messages between servers and clients. It handles moving your email from your client to the recipient’s mail server, hopping server to server if needed until it lands.
SMTP is over 40 years old. It was built for a time when email and the internet were far simpler. But it has been updated over the years for better security, efficiency, and reliability.
Despite its age, SMTP is still the foundation that email delivery is built on.
How SMTP Sends Emails
SMTP moves a transactional email in a few steps:
- Submission: A client (your email program or app) hands the email to an SMTP server for outgoing mail.
- Routing: The server works out the recipient’s domain and finds the right destination server using DNS.
- Transfer: The email passes from the sending server to the recipient’s SMTP server, sometimes through several servers along the way.
- Delivery: Once it reaches the recipient’s server, a local protocol like IMAP or POP3 drops it in the inbox.
Advantages of Using SMTP
SMTP is the go-to for most casual senders of transactional email:
- High availability: It’s supported almost everywhere, so it works across different systems and apps.
- Simplicity: The protocol is straightforward and needs little configuration.
- Reliability: SMTP servers queue emails and retry if the first attempt fails, which keeps delivery rates high.
Cons of Using SMTP
SMTP has limits too, especially for high volumes:
- Fewer features: It lacks the real-time analytics and dynamic content that APIs offer.
- Spam exposure: Its open, simple design has been abused for spam. That’s why authentication like SPF, DKIM, and DMARC matters.
- Performance: At high volume, the queueing and retries can make SMTP slower than an API.
Even with those limits, SMTP is a reliable, universally compatible way to send transactional email. With modern authentication layered on top, it’s likely to stay the most common method for years.
See our full guide for sending email over SMTP with SendLayer.
What Is an Email API?
An Application Programming Interface (API) for email is a set of instructions that lets an app, CRM, or other system talk directly to an email provider’s system.
Instead of configuring servers or wrangling protocols, developers use API calls to send email, manage contacts, and track performance.
How APIs Send Email
- Integration: First you connect the email API to your application or software.
- API call: When an event fires an email, like a user signing up or requesting a password reset, your app makes an API call. It includes the content, the recipient, and any instructions.
- Processing: The email service receives the request, assembles the email, and handles delivery to the recipient’s server.
- Feedback loop: Many email APIs send back real-time status, including delivery confirmations, bounce reports, and engagement metrics.
Advantages of Using an API for Email Delivery
Sending transactional email by API has several advantages over traditional SMTP:
- Speed and efficiency: APIs are built for performance and can send large volumes fast.
- Scalability: As your needs grow, an API scales to handle more traffic easily.
- Customization: APIs allow deep customization, from individual email content to conditional logic that targets specific user segments.
- Advanced analytics: Most come with reporting tools that show performance, engagement, and behavior.
- Reliability: Modern APIs have solid error handling and retries. And because API email goes over HTTP, it dodges the firewalls that block common SMTP ports.
Limitations of Using an API
An API can look like the obvious choice, but it suits power users and developers more:
- Complexity: Setting up an API takes more technical skill than SMTP.
- Cost: SMTP is often simpler and cheaper to start. API services can cost more at high volume or with advanced features.
- Dependency: An API ties you more closely to one provider, so choose your provider carefully to avoid service issues.
An email API is a modern, flexible option. It fits businesses building their own apps that want speed, scale, and detailed analytics.
See our guide to getting started with the SendLayer API.
When to Use SMTP
SMTP is the right choice more often than people expect. Reach for it when:
- You’re using a CMS or a plugin. Most WordPress plugins and website builders take SMTP details, not API keys. SMTP just works.
- You’re on a legacy system. Older apps and infrastructure almost always speak SMTP out of the box.
- Your volume is small and steady. For password resets, receipts, and notifications at modest volume, SMTP is simple and dependable.
If you want email working today with the least setup, SMTP is usually it.
When to Use an Email API
An API pays off once your sending gets bigger or more complex. Choose it when:
- You send high volume. APIs process bulk email faster than SMTP’s back-and-forth relay.
- Your emails are event-driven. If a signup, order, or in-app action triggers mail, an API call fits your code cleanly.
- You need real analytics. Opens, clicks, and bounce data come built in, so you can see what’s working.
- You want personalization at scale. APIs make it easy to tailor content per user with dynamic templates.
- You hit SMTP rate limits or blocked ports. API email goes over HTTP and sidesteps both problems.
If you’re building your own app and want control, an API is worth the extra setup. To see it in action, check out our step-by-step guides on sending email in Python, PHP, Vue.js, and with Firebase Functions.
How to Choose Between SMTP and API for Your Email
SendLayer offers both SMTP and API sending, at no extra cost. So which is right for you?
As a rule of thumb, unless you’re sending a lot of bulk email, SMTP works just fine. For most people sending transactional email with SendLayer, it’s easy to set up and widely supported.
If you send a high volume, an API may serve you better. Here’s why:
- API sending is more efficient. SMTP relay is a back-and-forth process that takes more communication with the receiving server, which can be slower.
- API email isn’t subject to the rate limits some SMTP servers impose.
- API is more flexible and powerful, with more automation and customization if you’re building your own app.
- API email can carry larger attachments, up to 25 MB versus 10 MB over SMTP.
Sending by API is usually more complex. But in some cases it’s actually quicker and easier than SMTP.
For example, if you use Easy WP SMTP for your WordPress emails, you can just enter your SendLayer API key instead of copying over all your SMTP credentials.
The same goes for automation tools. If you build workflows with a platform like n8n, you can connect SendLayer over SMTP or API, whichever fits your setup.
Still not sure which to use? Get in touch with our technical support team and we’ll be happy to help.
Frequently Asked Questions
Is an email API better than SMTP?
Neither is better overall. An API is faster and richer for high-volume, app-driven sending. SMTP is simpler and works almost everywhere, which suits most sites and plugins.
Is SMTP or API faster for sending email?
An API is generally faster at volume. SMTP relays each message through a back-and-forth handshake, while an API sends over HTTP in a single lightweight request.
Do I need to be a developer to use an email API?
For most API setups, yes, some coding is involved. But tools like Easy WP SMTP let you connect SendLayer’s API with just an API key, no code required.
Does SMTP or API affect email deliverability?
Deliverability depends mostly on authentication (SPF, DKIM, DMARC) and sender reputation, not the connection method. Both SMTP and API deliver reliably when your domain is properly authenticated.
Can I use both SMTP and API with SendLayer?
Yes. SendLayer includes both methods at no extra cost, so you can start with SMTP and move to the API later if your needs grow.
That’s It. Now You Know the Difference Between SMTP and API
Both methods send reliable email with SendLayer. SMTP keeps setup simple, and the API gives you speed and data as you scale. You can even switch later if your needs change.
Want to go deeper? Read our tutorial on how to check email delivery status to understand delivered vs. accepted email.
