Printer Friendly Version



This page was last updated: 10/24/2014 10:12:58 AM

I'm trying to send a large file attachment via email, but it's failing. Why?
I'm trying to send a large file attachment via email, but it's failing. Why?

Internet email systems are neither designed nor intended to move large files around. They are designed for sending email, and optionally, an attachment of reasonable size. Unfortunately, there is no set industry standard on maximum message size on the Internet. Every company, organization, and Internet email provider is free to set their own limit. Many Internet email providers set the message size limit in the 10MB range (e.g. Hotmail). Some, including Yahoo, AOL, and Gmail, are a little more generous and allow 25MB messages.

Note: the size limit refers to the entire message, not just the attachment. Due to the overhead created by MIME attachment encoding and other factors, the actual maximum attachment size that you'll be able to send is usually somewhat less than that limit. In other words, a mail system that allows 10MB messages will handle attachments roughly in the 8MB range.

Our CWSL Exchange email system allows messages up to 30MB internally as well as out through our email gateway to the Internet. However, that doesn't mean that the email system on the other end will accept that message, because there is no established standard across the Internet for maximum message size. Therefore, there should be no expectation of success when sending a large file via email, particularly if the file is larger than 10MB. A good rule of thumb is that anything less than 10MB is likely to be OK when sending over the Internet as an email attachment. If it's larger than that, it can be hit or miss, so have no expectation of success of sending an attachment larger than 10MB out to the Internet unless you know that the recipient's email system supports an attachment of that size.

The issue of Internet email message size limit has created an opportunity for 3rd-party cloud-based systems to handle the sending of large files over the Internet. Some that come to mind are,,, and These are just a few. Just Google send large files, and you'll find at least a dozen sites that offer this service, some for free or with a trial account, or with a limitation on size. The general idea of these services is that they offer a cloud-based temporary file storage location. The sender uploads the file and is then given a URL to that file. The sender can then email this URL to whomever needs to download the file. After X days the service will delete the file from their system, so they are not accumulating storage obligations. These services are simple to use and allow the transfer of huge files over the Internet without ever impacting an email system because the file itself is never emailed, only the URL is, which is of course very small. Using one of these services is the recommended way to send large files to someone else on the Internet.

If the message you are sending (or expecting to receive) does not contain a large attachment, it could be that the attachment is of a type that is blocked at the gateway. Many file types, including executables (.exe, .com, .vbs, etc.) and zip files, can be malicious. These are blocked without notification to the sender or recipient by the CWSL email gateway. Again, when sending large attachments, or any attachment which could potentially be malicious, we recommend using a file transfer service, like those mentioned above.

In summary, when trying to send a large file to someone else on the Internet, do not use email. Use one of the aforementioned 3rd-party services. If you are trying to send a large file to someone else on the CWSL network, use a folder on our file servers. Contact HelpDesk if you are not sure which folder is appropriated to use.


     © Copyright -- 2016 -- California Western School of Law -- All Rights Reserved