API vs. SMTP in 2024: What’s Right for Deliverability?

I doubt there’s a single business out there that doesn’t send out hundreds or thousands of emails a month to potential and existing customers. 

Sure, social media is a great way to connect with your audience and build trust and credibility, but there’s no better way to execute your sales and marketing efforts than with emails, especially for B2B companies. 

Most companies gear up with special sales or marketing tools like Reply.io to automate and scale their email outreach, leaving it up to them to ensure everything runs smoothly in the background, and the emails reach their intended recipients’ inboxes, aka email deliverability. 

However, huge email service providers like Gmail, Yahoo, and Outlook don’t make it easy. 

In their defense, there are tons of malicious, non-solicited, and spammy emails being sent each and every day, and it’s the responsibility of the email providers to proactively flag such emails and prevent them from reaching their users. 

The only problem—it’s difficult for them to ensure they’re keeping the ‘bad’ emails at bay but letting business sales and marketing emails go through, 100% of the time. 

In this article, we’ll explore and compare the two main methods for sending emails, SMTP and API, and determine how they impact deliverability in the current landscape. 

Why all the fuss over email deliverability? 

If you’re a B2B professional, chances are you’ve come across the topic of email deliverability at least once or twice over the last couple of years, and you may be wondering, why all the fuss?

Well, considering that emails are the primary communication channel for sales and marketing teams, ensuring they get delivered to the right person at the right time may very well be the difference between a new customer and a lost opportunity. 

And that’s just one email. 

In the context of thousands, email deliverability can directly impact lead generation, conversions, retention, and ultimately—revenue. This is even more painful when you think about all the time and effort sales and marketing teams invest in building targeted email lists, crafting personalized messages, and developing lead-generation strategies. 

Gmail has ramped up its email security policies lately, and businesses that are not cautious run the risk of investing time and resources in their customer communication, only to have a good chunk of those emails never reach the intended inboxes. 

Hence, the fuss.

The new 2024 mass-email policies 

At the beginning of the year, email giants like Gmail and Outlook changed the way individuals and businesses are allowed to send large volumes of emails without getting flagged and blacklisted. 

As a result, in the context of the number one email providers like Gmail, API vs SMTP became a trending topic of conversation due to increasing compliances affecting deliverability.      

And just as we all slowly got on board with these changes, Gmail recently revealed its brand new email-sending policy—open tracking pixels.

Open tracking pixel flagging

To better understand how this works, think about the outreach tool your team or business uses for email outreach (like Reply.io). I can almost guarantee that, among other features, they have the ability to track crucial metrics like open and reply rates. 

The only way they can do that is by attaching special open tracking pixels to each and every email that gets sent through the platform, which then allows outreach tools to determine whether they were opened and/or engaged with in any way. 

Well, long story short, Gmail will now flag such emails in an attempt to further enhance user privacy, increasingly showing users this giant ‘report spam’ button once it catches these open pixels:

Stricter spam rates and authentication requirements

Going back to early 2024, Gmail shocked the world with two major security updates: mandating mass-email senders to keep spam rates below 0.3% and ensuring that all authentication protocols (DMARC, SPF, and DKIM) are 100% correct and compliant, both having huge impact on your deliverability rates.

At Reply.io, we quickly added API integrations with Google’s Postmaster and authentication protocol checks, taking this tedious process off our users’ shoulders while ensuring full compliance at all times:

Now that we’ve covered the basics of email deliverability and the new challenges businesses are facing, let’s take a look at how API and SMTP protocols can keep up with them. 

Understanding the basics: API vs SMTP

SMTP (Simple Mail Transfer Protocol) and API (Application Programming Interface) are the two main methods that everyone can use to send emails, allowing servers and applications to connect with email servers and deliver the intended message. 

They are two distinct methods with unique characteristics and use cases.

What is SMTP? 

SMTP is the traditional and straightforward communication method that simply handles email transmission between servers, nothing more, nothing less. 

This email protocol is universally supported across most email software while being pretty easy to implement and operate, which is the main reason why so many businesses have used SMTP as their go-to email protocol since the beginning of the internet. 

However, being the traditional method has its costs. 

SMTPs are far less capable with regard to email tracking and automation, which, as you can imagine, are both crucial for modern sales and marketing teams. On top of that, due to the lack of advanced automation, SMTPs send your emails through multiple digital ‘hurdles’ before they reach their intended recipient, which may harm time-sensitive emails. 

They’re more than okay for the more simple email-related processes like small automated campaigns and newsletters, but probably won’t be sufficient for high-volume campaigns and established businesses.

Because SMTP protocol is considered a more ‘manual’ way of sending emails, it’s important to clearly understand what is SMTP server and how it works to ensure smooth deliverability. 

What is an API? 

Meanwhile, APIs are a more modern and versatile method that facilitates email transmission directly through an email service provider while allowing multiple software to communicate with each other. 

Due to their flexibility, APIs provide several advanced features and integrations that, for example, track emails, manage email lists and inboxes, schedule messages, and the list goes on and on. 

When set up correctly, APIs offer more advanced features than SMTPs, such as real-time analytics, action-triggered emails, inbox management, and other useful features that help businesses shape their sales and marketing strategies. 

As you can imagine, they’re slightly more technically complex, so at times, APIs may be tough to implement and regularly maintain, though in all fairness, the top email outreach tools do most of it on their own and provide assistance along the way. 

APIs are the right choice for businesses working with high-volume email campaigns, requiring more advanced automation to effectively set up, monitor, and optimize their sales and marketing outreach. 

How API and SMTP impact email deliverability

Now that we’ve covered the basics of API vs SMTP email protocols, time to explore how they both impact email deliverability to help you determine which is better for your business. 

When it comes to compliance, which is one of the ‘big three’ of email deliverability policies, APIs offer native, automated authentication and configuration processes, significantly reducing the likelihood of errors. 

SMTP configuration can also get the job done, but more often than not, it’s a manual process, which makes setting up and maintaining authentication protocols more challenging and risky. 

Both SMTP and API can meet the modern security requirements of large email providers, the only difference is that API provides a more seamless experience, along with stronger encryption, but that’s a whole other topic. 

Yet, turns out, sending emails via API can get you 10-15% more opens compared to SMTP! On average, API emails see a 42.4% open rate, while SMTP lags behind at 30.5%.

Source: 2024 Cold Email Deliverability Guide by Reply.io

SMTP has always been a multi-step email delivery system, meaning it has to pass multiple checks and get all the boxes ticked before the emails land in their designated inboxes, which can sometimes be suboptimal for businesses. 

On the other hand, APIs are built with speed and efficiency in mind, so if we’re talking about the speed of email deliverability, API takes the crown once again. 

With regard to the new pixel tracking update, both SMTP and API protocols most often include such pixels in their emails, so both run the risk of getting flagged and sent to spam, the only difference being that APIs also offer alternative methods for measuring email performance (click-through rates, for instance). 

Given that these pixels can be really harmful, it’s probably worth mentioning that we may be entering a new era where businesses should possibly look into other ways of measuring the success of their email campaigns other than open rates (reply rate is the best option here).  

All in all, SMTP is more prone to email deliverability issues, especially in mass email campaigns, whereas API offers a faster and more reliable email-sending process due to its streamlined authentication protocols setup and maintenance, as well as versatility with third-party integrations, so it’s no surprise that it has slowly become the preferred choice for modern businesses. 

That said, it’s worth noting that both SMTP and API may serve different purposes depending on business email communication needs. 

SMTP may be the more cost-effective and simple choice for individual professionals and small teams, whereas API delivers higher-level functionality that’s more aligned with the needs of growing organizations. But, getting access to Google’s API isn’t easy.

Developers have to pass a tough Google verification every 12 months, ensuring the email tool uses the API securely and correctly. Tools like Reply.io that pass this verification build more trust with Google, which can lead to better deliverability.

So, here’s a quick win—opt for sending your cold emails via API. Here’s how you can get started:

Another important thing to note—many cold outreach tools don’t support sending emails via API, making Reply a great alternative to those options!

As a matter of fact, selecting the right protocol empowers businesses to not only improve deliverability but also better manage their customer engagement and maintain higher levels of email performance.

SMTP vs API — which is better for your business? 

Deciding whether SMTP or API is the way to go for your business is entirely based on how much email volume you work with, and how important emails are to your overall operations. 

SMTP can be a good choice for smaller businesses and startups that have an email list of a couple hundred, and only use email for their weekly newsletter and occasional promotions. 

If that’s the case, and you don’t require any advanced email performance tracking, and you have someone with IT experience to ensure correct authentication and compliance at all times, then SMTP is a simple and cost-effective way to send your business emails. 

On the other hand, API protocols are the superior option for businesses seeking to scale and automate their email operations. If your business works with high-volume sales or marketing email campaigns, APIs will provide real-time data for effective decision-making, advanced automations, and last but not least—built-in compliance features that will protect your email deliverability from that front. 

Businesses that work with emails for their prospecting, engagement, and nurturing will more often than not gear up with a reliable outreach tool like Reply.io, which leverages native APIs with email providers and other tools to ensure users can just focus on the messaging, while the software takes care of all the rest. 

At Reply.io, we put deliverability at the top of our priority list in 2024 to ensure all our users’ email campaigns don’t suffer in light of all the next regulations and compliance requirements — we’ve added new spam tracking and authentication integrations, assembled a dedicated deliverability team to help troubleshoot any issues, and continuously help educate our audience on the current trends and best practices in email deliverability.

Understanding the role of port 587 

I know what you’re thinking, now there’s some weird sort of 3-digit port? 

Well, if your business is exclusively using APIs to handle all email communications, you may as well skip this part, but if you’re sticking with the traditional SMTP (or working with both), then understanding what these ports are and how they work may be of value, especially in the context of deliverability. 

In simple terms, port 587 is the golden standard for sending emails securely via SMTP. 

While APIs handle everything to do with security and encryption under the hood, without the need for users to worry about a thing, SMTP protocols are not as modernized, which is why they use these special ports. 

Port 587 is particularly responsible for email delivery by ensuring that all outgoing emails are properly authenticated with key protocols like SPF, DKIM, and DMARC. Given the new ramped up security rules in today’s email environment, these authentications have become paramount to ensure your emails reach their audience and don’t end up in spam. 

For many years in the past, port 25 was the default email-sending port for SMTP users, however, it has now become quite obsolete given the growing importance of privacy and security, which port 587 does a much better job with. 

The main value point of port 587 and the reason it’s the most preferred choice among modern businesses is that it supports TLS encryption—the upgraded version of encryption technology that’s pretty much flawless. 

Therefore, port 587 provides a thick extra layer of security to protect your email content, ensuring your emails not only comply with all regulations but also stay safe during transit.  

The second most common port that SMTP users may come across is port 465, which, unlike 587, sticks with SSL encryption, a less strict and secure technology than TLS. So while for simple, low-volume, low-impact emails port 465 will get the job done, port 587 should be the preferred option for businesses, because with higher email volumes come higher expectations. 

In Gmail, for example, users can simply enter their desired port number when configuring their sending settings, so while it’s not that complicated to set up, it’s sure important to understand the different ports. 

The shifting tides of email deliverability 

All these security policies have the potential to significantly harm your deliverability, and the fact that Gmail keeps rolling out new ones shows that this is far from the end. And while battling spam and malicious email should be celebrated, unfortunately, this comes at the expense of businesses attempting to engage with their audiences via email.   

Thus, the choice between SMTP and API has become a more common area of discussion. 

In a nutshell, though SMTP offers a simpler and more widespread email-sending service, APIs are more aligned with the modern business landscape that is built on speed and automation, providing superior authentication and compliance, as well as more versatility in light of new rules coming to life. 

To keep it completely honest with you, with things around email deliverability getting much more complex by the month, it’s better to gear up with a reliable outreach tool that puts email deliverability at its forefront, just like we do at Reply.io. 

That way, your outreach platform will take care of everything to do with email deliverability, ensuring all your authentications are compliant at all times while monitoring spam rates and automatically making any necessary changes, so your team doesn’t even have to worry about all these complex technical issues. 

Evaluate your current email infrastructure and consider whether transitioning to an API-based system could help you achieve better deliverability and compliance in light of the latest industry changes.