If you run e-commerce of any size, are an influencer, or simply sell something online, you must feel that your transactional emails are sent smoothly. The good news is that your choice will come down to two main options: Web API or SMTP. On the other hand, each of the variants has its strengths and weaknesses.

The deciding factor is often a combination of convenience for your use case, availability of providers, coding cost, operational resources, and the relative priority of things like delivery speed and ease of migration to your business. Let's compare these conditions for sending an email using SMTP and API.

What is the difference between Web API and SMTP Relay?

SMTP stands for Simple Message Transfer Protocol. It combines the set of established procedures and principles to be used in sending transactional emails to an email server. Forwarding letters by mail agents rely on specific IMAP and POP3 email protocols for receiving and sending messages between servers. Such a transfer involves multiple communication (relaying) between servers using commands and codes.

API (Application Programming Interface) is simply a means by which different applications, platforms, software, and code communicate with each other and share resources. It uses the capabilities of hypertext protocols (HTTP) to connect with a mail agent, making the process more universal, safer, and faster.

That seems pretty straightforward, doesn't it? That's because it is and can be shown in the form of a table:

Pros and cons of SMTP relay

For starters, SMTP is the more widely adopted of the two, as you don't have to worry about it in terms of integration and usage. All devices capable of sending emails have an SMTP extension. The SMTP connection requires little or no knowledge of coding practices, as its main focus is simply changing the current outgoing server to the new one.

Along with the evident benefits of using SMTP come some problems worth mentioning. The SMTP connection is a conversational protocol, which means numerous round-trip communications with the server, which adds time to longer mass mailings. At the same time, it does not allow great versatility and flexibility compared to integration via REST API. As you can see, although the SMTP makes it easier to make the connection, as it is a conversational protocol, it has certain probabilities of error and lower performance compared to large-scale shipments.

Let's see next if the API solves these problems.

Pros and cons of Web APIs for email sending

The REST API makes sending transactional emails easy and automates processes through your application and/or platform. APIs, in general, have better delivery speeds and can be executed in a matter of seconds. Not to mention that sending using SMTP will be a slow process, but the API can make it faster. It also reduces the possible steps to fail when sending a transactional email.

The API also adds an extra level of security with an API key. This added level of protection is essential if you process and send highly sensitive information via email. Since the APIs use HTTP, most firewalls allow it, except for a few that have particular restrictions.

Things are looking good for API; however, there are some drawbacks. The main disadvantage of the API is that it requires some programming knowledge to configure and execute transactional sends. It can be costly in terms of time spent if you don't know how to use its libraries. The detailed description of sending emails with URLs is not easy for the uninitiated. Therefore, in these situations, the SMTP connection may be the solution.

Another drawback of the API is that it can change over the years. They tend to require constant updates and modifications to keep up with changes. The SMTP connection, in comparison does not have this problem since it is a standard that will be used to send transactional emails.

SMTP relay vs. Web API – an attitude to email testing

Both protocols can be in hand if you are concerned with checking that your emails have the correct HTML code, are free of errors, and are delivered to the right place. As with the process of sending a letter, testing the SMTP Relay mailing is much clearer. In addition, there are many automatic tools for testing SMTP servers based on Mailtrap, Mxtoolbox, or SendGrid, capable of conducting a comprehensive check of your mailing list.

The exact process with Web API will take longer due to the extended test parameters. As already mentioned, working with the API implies handling code with a choice of sequences, input and output values, and so on. You can get professional testing assistance from Postman, cURL, and Soatest tools.

What should I use: SMTP relay or Web API?

Many users and providers will give different answers to that question. If you are tech-savvy, using APIs is a more attractive option as it allows you to customize and modify settings to suit your needs. There is even a Solomonic decision to choose Web SMTP API integration. In this way, predictability and manageability can be combined with an option to track and store messages in an internal CRM or an external system.

In general, the option to use SMTP or API is entirely different and depends on your needs and the capacity of your application, platform, eCommerce, or website. Despite the earlier advent of SMTP and the more modern universality of the API service architecture, both protocols have their adherents.

To wrap up

So, choosing the right email sending method depends on several factors. A cloud-based SMTP service could be just as powerful as an HTTP API. Yet, an API could offer a bit more flexibility in the level of customizations, faster times, and a relatively low chance of failure due to verification processes.

If you send standard transactional e-mails and do not need advanced functions, you can freely choose integration via SMTP. If you need to deliver a lot of messages, e.g., tens of thousands of e-mails a day, or look for more advanced functions, e.g., personalization or template selection, then integration via API will be a better solution for you.