Each email sending provider has its own maximum email size limit. however, there is no single standard: limits are dictated primarily for performance and security reasons. There are many things to consider when preparing your email notifications or newsletters: from the maximum size of attachments and the html part, to the email client your recipients are likely to use. Plus, there are some not-so-obvious factors that affect actual email size and deliverability. In this article, we have collected answers to the most common related questions:
- what is the recommended size for email body and attachments?
- what are the limits of different email clients and delivery providers?
- how do they affect embedded images? email size, spam score, and deliverability?
- what are common email size issues and errors and how to handle them?
Also, we’ve implemented an email size limit for mailtrap – let’s review how it works and how it can help you test your email deliverability.
types of email size limits
Email size limits are used for three main reasons:
- performance. email systems are not designed to transfer large volumes of data. they deal with smtp, which is a pretty deprecated protocol. most systems cannot process multiple data sources at once. in addition, mail transfer agents often scan incoming data for viruses and other potential risks, which also consumes resources and affects system performance.
- base/storage size of data. email data must be stored on a server (either cloud or local). so not only the message itself, but also the inbox as a whole has its own limits.
- security. Items 1 and 2 on our list also lead to potential security vulnerabilities. email systems typically use a limited number of ports to retrieve data. this makes them a candidate for dos (denial of service) attacks: a large number of large messages can bring down the entire system. this is also known as an email bomb.
When preparing marketing or transactional emails, there are two limits to keep in mind:
- limit to send a message (your email system)
- limit to retrieve a message (your recipients’ mail system).
For example, if your email service can send messages up to 15mb but the recipient of the email can only retrieve messages up to 10mb, then obviously your email should not exceed 10mb. otherwise the email will be rejected. it’s helpful to know the mapping for your recipient’s incoming messages.
If you work with a large database of email addresses to send transactional or marketing emails and you don’t have those details about your recipients, you should follow one rule of thumb: send messages no larger than 10mb .
limits for popular email sending clients and providers like gmail, outlook, sendgrid, etc.
You can see that most email clients are capable of retrieving 20-25mb of email data. it is quite a rare situation when you need to send a mass email with large attachments. but it is still possible when you send numerous confirmations of reservations or orders, for example. however, in addition to general deliverability, there are a few other concerns related to sending large messages. spam scoring and rendering issues are the most important. also some esp like postmark have additional limits for email body size.
what else should you consider about your email size?
When taking care to create a message that will be successfully delivered to your recipients, you also need to think about which folder it will go in, how quickly it will be delivered, and how happy your recipients will be.
check your emails
spam score
Avoiding spam filters and targeting the main folder of your recipient’s inbox is one of the trickiest things in email management. you will not find strict and ubiquitous criteria for spam detection. however, there is a list of common “anti-spam” rules and size-related bullet points at the top.
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 to be 5mb each, while gmail trims the body of messages larger than 102kb.
Many tests by various vendors demonstrate the correlation between email size and deliverability. In most cases, messages with a body of more than 100 – 150 kb trigger spam filters. therefore, the common recommendation is to keep the email body size between 15 kb and 100 kb.
rendering problems
Another aspect of large emails is their loading or processing speed. Your recipients don’t always have access to a high-speed connection, and a long message upload process can cause irritation. furthermore, in many email clients, a large email body may only be partially displayed, with your recipient seeing only parts of images or cropped content. this could severely damage your customers’ digital experience and leave an overall bad impression.
Despite many hidden limitations and restrictions, there are common rules for creating high-performing emails. the first of these is to reduce and optimize the size of the email body.
what affects email size and how can it be optimized?
Each element of the message and how it is implemented affects the size of the email.
- attached files. with attachments everything is simple. these are attachments and do not affect the size of the email body. the total email limit generally includes all attachments. the best option is to upload the files you need to transfer to the cloud and put a link in your message. due to security concerns, many file types are blocked or considered dangerous (including .doc, .xls, and .pdf).
- images and other embedded elements. Even simple notifications often contain images, at least your company logo. its size depends on the format of the image and the way the image is embedded in the message. We take a closer look at this topic in this article. in short, whenever possible, link to an image hosted on an external server (in the html, so that it is displayed as part of the message).
- encoding. mime- encoded messages (base64) add 33% to the size of your email. keep this in mind when encoding a multipart message.
- other. each element of the message affects its overall size. message headers, signatures, formatting – these all add extra kilobytes.
rules of a perfect email
Keeping in mind the limits and dependencies mentioned above, we’ve created a short email size optimization checklist. Here are the best practices for creating an email that works well in terms of size:
- keep the email body from 15kb to 100kb.
- do not add attachments larger than 10mb. if possible, upload your files to the external server and make them available online or for download via a link.
- do not abuse images, gifs and other multimedia content.
- remember that almost 42% of emails are opened on mobile devices. keep this in mind when creating email templates. They should be responsive. using a responsive email framework or email generator can save you nerves and kilobytes.
check email size in mailtrap before sending
Even if you followed all the recommendations and have created the perfect message, checking its size is not an easy task. Whether you’re encoding a message in your app or preparing a template to send via gmail, you need to use a third-party tool, send your message, or download it to scan its size.
If you use mailtrap to test your emails before sending them to your actual recipients, you can easily check both the email body and the size of the attachment.
As soon as your test message is stuck in your virtual inbox, the full size of the email is displayed along with the message headers. if your message has attachments, you’ll 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 5MB (total email size with attachments) for free. the maximum possible email size for testing is limited to 25mb.
smtp errors related to email size
Instead of offering a conclusion, let’s review the most common smtp errors related to mail size. there are two of them:
The smtp codes will be filled with additional numbers and messages so you can understand the exact reason. The general rule of thumb is to keep your email size as compact as possible, verify and test it before sending it to real recipients.