SMTP vs API - Decoding the Two Ways of Sending Transactional Emails

Transactional emails are a must-have business communication tool in the modern age. These messages provide crucial information to customers, such as transaction notifications, order confirmations, account logins, and more. From banks to eCommerce stores and streaming services, businesses from various industries integrate email functionality into their CRM platforms to send messages triggered by specific customer actions or interactions.

However, the method of connecting their systems with emailing software to send transactional emails can impact their effectiveness.

In this post, we’ll share the differences between Simple Mail Transfer Protocol (SMTP) and Application Programming Interface (API). Our insights will allow you to determine which technology suits your needs, capabilities, and budget.

What is SMTP?

Simple Mail Transfer Protocol, or SMTP, is the oldest email method and has existed since the late ‘80s. It’s essentially a postal service for digital messages. It transfers emails from the sender’s server to the recipient’s inbox.

How to Use SMTP to Send Transactional Emails

Businesses connect to an SMTP email service using their email platform’s TCP connection to send transactional emails using SMTP. The SMTP server validates the email and user credentials using preset instructions and response codes. The email is relayed to the recipient’s email platform’s SMTP server if everything checks out.

Pros and Cons of SMTP

Now that you understand what SMTP is, here are its main benefits and drawbacks for businesses looking to use it to send transactional emails:


1.     Simplicity and Ease of Use

SMTP is known for its simplicity and ease of use. You can configure your email client or server with your credentials, and it will work seamlessly for sending and receiving emails. This straightforward setup makes it accessible to many users with limited technical expertise.

2.     Simple Troubleshooting

When an SMTP email fails, the system generates a bounce-back message that explains the failure, such as invalid address, mailbox full, etc. These explanations can be invaluable for troubleshooting email delivery issues.

3.     Rapid Deployment of Transactional Emails

SMTP is excellent for sending transactional emails, such as password resets or order confirmations, making them ideal for banks, retail stores, and other businesses that rely on real-time notifications.


1.     Firewall Interference

Some network firewalls can block the port commonly used with SMTP (Port 25). This limitation can pose challenges for users operating within network environments with strict security measures. The good news, however, is that many email services offer alternative ports like 2525, 587, 465, etc., to avoid blockage.

2.     7-Bit ASCII Characters

SMTP is primarily designed to handle 7-bit ASCII characters, which can restrict using non-ASCII or extended character sets in emails. This limitation may result in encoding or character substitution for special characters or non-English language content, potentially affecting the readability and accuracy of messages.

3.     Relaying Delays

SMTP often involves multiple relays and conversions between email servers to deliver messages from the sender to the recipient. This process can introduce delays in message transmission, which may affect the real-time delivery of critical emails.

What is Email API?

API is an acronym for Application Programming Interface. Unlike SMTP, which is used specifically for email, APIs can connect various services or applications with a system using Hypertext Transfer Protocol (HTTP). In that context, an email API allows your website, app, or CRM system to communicate with email servers and services, enabling direct email functionality.

How to Use Email API to Send Transactional Emails

Utilizing an Email API for sending transactional emails involves a technical integration within your application or system. The Email API serves as a bridge between your software and the email service provider’s servers. When you trigger an email send, the API communicates directly with the email servers, eliminating the need for relaying through your server. This real-time interaction enables instant email delivery.

Pros and Cons of Email API

Now that you understand what Email API is, here are its main benefits and drawbacks when sending transactional emails:


1.     Faster Delivery

APIs offer faster email delivery times due to less back-and-forth communication for approval. Faster delivery can be particularly crucial for real-time communication or transactional emails where timeliness matters.

2.     Access to Valuable Data

APIs offer access to crucial statistics related to email interactions. Businesses can benefit from this feature as it provides insights into recipient responses, helping them refine email marketing strategies.

3.     Simplified Maintenance

Opting for email API means you integrate a third-party email service into your system or site. Hence, you’re responsible for coding or service maintenance.


1.     Expertise Requirement

Businesses using email APIs require moderate or high IT expertise to manage integrations with email services. If you lack the necessary skills, configuring and utilizing email APIs effectively can be challenging.

2.     Version Control and Updates

APIs require regular version control and updates. Over time, changes and modifications are essential to keep the API current and compatible with evolving technologies. In contrast, SMTP is a standardized email-sending protocol that does not face the same challenges of version control.

SMTP vs. API – Which One Should You Choose?

Can’t decide which technology works for your business? Here are five factors to consider that can help simplify your decision:

1.     Compatibility

If your website or business application cannot integrate APIs for email sending, SMTP becomes the preferred choice.

2.     Automation

APIs offer better automation capabilities if you operate an online business and need to send bulk emails regularly.

3.     Expertise

If you possess technical expertise, utilizing APIs becomes more appealing since it gives you the flexibility to tailor and adjust configurations according to your requirements.

Final Word

Transactional emails are indispensable to effective business communication. Hence, the choice between SMTP vs. API is not as straightforward as you’d imagine. However, as a business, you should prioritize email delivery speed, security, and consistency when choosing your preferred method while assessing your capabilities, preferences, and budget. Your choice will impact the effectiveness of your email communication strategy.

Maileroo is a powerful email platform offering both SMTP and API methods for sending transactional emails. Feel free to learn more about our services and enjoy cutting-edge features like email automation and real-time performance tracking.