What is the maximum size of email attachments?

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While our email system currently supports a maximum message size of 5MB this doesn't necessarily mean that one can attach and send files up to 5MB. Although the size of files which can successfully pass through our system attached to email will vary as a general rule of thumb one should be able to send files up to 3MB-3.5MB or so in size without encountering any problems.

If the maximum message size is 5MB why can't I send an email with a 4.9MB file attachment?

This is due to the process referred to as MIME encoding. Email was originally intended solely for and is primarily designed to handle text messages. What makes it possible to send binary files (non-ASCII text files such as program setups, multimedia files, images, compressed archives, etc) over the Internet attached to email is MIME encoding.

The MIME encoding/decoding process supported by all of today's popular email clients essentially takes binary files, converts them to a plain text format for their trip across the web and then decodes or reconverts them back to their original usable binary format once the email reaches it's destination.

While MIME encoding is what allows binary file attachments to be sent via email in the first place one downside to the process is that it isn't particularly efficient storage-wize. As a general rule MIME encoded binary file attachments are approximately 150% of the size of the original pre-encoded file, i.e. a 3MB *.zip or *.exe file can easily exceed 4.5MB once encoded and attached to an email.

As a result of MIME encoding the maximum size of non-text file attachments which can be successfully pass through our email servers will commonly be somewhere around 3MB to 3.5MB.

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