E-mail message size limits

You attempt to send an e-mail with a large attachment, or someone outside the university attempts to send you an e-mail with a large attachment, and the message is rejected due to size.

E-mail was designed to efficiently transmit text and was never intended nor designed to transfer large files. As a result, large attachments put a strain on e-mail systems. Therefore, many e-mail systems will simply not accept large messages, some with a threshold as low as 5 MB. The UT Health Science Center's central e-mail systems will accept messages-- both inbound and outbound-- of up to 30 MB.

Keep in mind that although UTHSCSA's limit is 30 MB, other e-mail systems may have a lower limit than ours; the lowest limit along an email's path prevails.

Why is there a 30 MB size limit? 
E-mail was not designed to transmit large blocks of data. Rather, it was designed for small, text transmissions. This limitation is inherent in the underlying e-mail protocols that were developed several decades ago. (The method for including attachments used today was developed in the early 1990s and actually converts the binary file into text "gibberish" for transmission. The recipient's e-mail program then converts it back into its original binary format.) Consequently, the transmission of large files via e-mail places a disproportionate burden on e-mail systems and can also tax antivirus and antispam filters, which can cause general e-mail delays. Therefore, a reasonable size limit must be imposed in order to prevent our e-mail services from being overwhelmed. Additionally, the size limit helps to prevent possible denial-of-service attacks against our e-mail system.

Our 30 MB limit is about average; many organizations have limits lower than ours, some as low as 5 MB.

My message is less than 30 MB, so why am I getting the "message too large" error? 
The 30 MB message size limit we advertise is the actual total size of the entire message during transmission, including the body and all attachments combined.  However, it's not as simple as adding up the size of the message and all the attachments.  All attachments must be converted to text for transmission as the underlying e-mail protocols for Internet mail were designed for the transmission of text only, not for binary files. After this conversion and associated overhead, attachments can be up to 150% of their original size. This results in a de facto size limit of about 20-22 MB per message.  (The recipient's email client then converts the text attachment back to its original format upon receipt.)

Another possible issue is that the recipient's e-mail system has a lower size limit than ours. For example, if your message is 15 MB and the recipient's e-mail system has a 10 MB size limit, your message will make it through our system but will be rejected by the recipient's e-mail system.

Internally, our broadcast email service (the server that handles the large mailing lists such as faculty-staff, students, residents, and for the various schools and programs) has a message size limit of 10 MB.  This is because of the sheer volume of messages it will send at one time-- a message with a large attachment sent to that many people at once can potentially cause issues.

The 30 MB limit is a system-wide limit and we are not able to grant individual exceptions. Therefore, if you need to transmit something in excess of the 30 MB limit, you will have to use an alternate method. There are a number of options such as using a dedicated file sharing application, or sending the file via physical media. Which option you chose will depend on a number of factors, not the least of which is security. As a result, UT Health users should contact their TSR or the Service Desk at (210) 567-7777 for assistance on the various options.

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Article ID: 68378
Thu 12/6/18 10:49 AM
Tue 3/14/23 5:24 PM