Every email sending provider has its own maximum email size limit. However, there is no unique standard – limits are dictated mostly by performance and security reasons. There are many things you should keep in mind when preparing your email notifications or newsletters: from the maximum size of file attachments and HTML part to the email client your recipients will most likely use. Also, there are some not so obvious factors, which impact the actual email size and deliverability.
In this article, we have gathered replies to the most common related questions:
Also, we have implemented an email size limit for Mailtrap: let’s review how it works and how it can help you with testing your email deliverability.
Email size limits are used for three main reasons:
When preparing either transactional or marketing emails, you should consider two limits:
For example, if your email service is able to send messages up to 15 MB but the email recipient can retrieve messages only up to 10 MB, it’s obvious that your email should not exceed 10MB. Otherwise, the email will be rejected. It is useful to know the allowance for your recipient’s incoming messages.
If you work with a big database of email addresses to send transactional or marketing emails and don’t have such details about your recipients, you should follow a general rule: send messages that don’t exceed 10 MB.
You can see that most email clients are capable of retrieving 20 – 25 MB of email data. It’s a pretty rare situation when you need to send a mass email with large attachments. But it is still possible when you send numerous booking or order confirmations, for example. However, besides general deliverability, there are few more concerns related to sending large messages. Spam score and rendering issues are the most important. Moreover, some ESPs like Postmark have additional limits for the email body size.
Taking care to create a message that will be successfully delivered to your recipients, you should also think of which folder it will go to, how quickly it will be delivered, and how happy your recipients will be.
Avoiding spam filters and targeting the main folder of your recipient’s inbox is one of the trickiest things in email handling. You won’t find strict and ubiquitous criteria for spam detection. However, there is a list of common “anti-spam” rules, and the size-related points are at the top of it.
Technically, there is no message body limit in the SMTP protocol. Still, some providers have their own limits. For example, Postmark allows the HTML and text parts each to be 5 MB, while Gmail clips the body of messages over 102 KB.
Many tests by various providers prove the correlation between email size and deliverability. In most cases, messages with a body over 100 – 150 KB trigger spam filters. So, the common recommendation is to keep email body size between 15 KB and 100 KB.
Another aspect of large emails is their loading and/or rendering speed. Your recipients do not always have access to a high-speed connection, and a long-lasting message loading process might cause irritation. In addition, in many email clients, a large email body might be rendered just partially, and your recipient will see just parts of images or clipped content. This might strongly harm your customer digital experience and leave a bad general impression.
Notwithstanding the numerous limitations and hidden constraints, there are common rules on building well-performing emails. The first of these is to decrease and optimize email body size.
Each element of the message and the way it’s implemented impacts email size.
Considering the above mentioned limits and dependencies, we have created a short email size optimization checklist. Here are the best practices for building a well-performing email in terms of size:
Even if you followed all recommendations and have crafted the perfect message, checking its size is not an easy task. Whether you are coding a message in your app or preparing a template to send via Gmail, you need to use a third-party tool, actually send your message, or download it then to explore its size.
If you use Mailtrap for testing your emails before sending them to your real recipients, you can easily check both the email body and attachment size.
As soon as your test message is trapped in your virtual inbox, the total email size is displayed along with the message headers. If your message has attachments, you will see a list of them with file names and sizes.
Also, Mailtrap has email size limits, which depend on your billing plan. You can test messages up to 5 MB (total email size with attachments) for free. The maximum possible email size to test is limited to 25 MB.
Instead of offering a conclusion, let’s review the most common SMTP errors related to the mail size. There are two of them:
SMTP codes will be completed with additional numbers and messages so you will be able to understand the exact reason. The general rule is to keep your email size as compact as possible, check, and test it before sending to real recipients.
Thank you for reading our guide on how to evaluate and optimize an average email size. The article was initially published in the Mailtrap blog by Andriy Zapisotskyi.