Large File Attachment Help

E-mail was designed to efficiently transmit text and was never intended nor designed to transfer large files.  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 e-mail systems will accept messages up to 30 MB.

Keep in mind that although UTHSCSA's e-mail system may accept or transmit messages up to 30 MB, other e-mail systems may have a lower limit than ours; the lowest limit 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.  Consequently, the transmission of large files via e-mail places a disproportionate burden on e-mail systems and also can also tax the network and antivirus and antispam filters, which can cause general e-mail delays.  Therefore, we must impose a reasonable size limit in order to protect 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; some organizations have limits as low as 5 MB.

  • How can I transmit attachments larger than 30 MB?
    If you need to transmit anything in excess of the 30 MB limit, you will need to use an alternate method.  There are a number of options such as putting the file on a website and sending a link, using a dedicated file transfer application such as FTP or a "dropbox" service, or sending the file via physical media.  UT Health Science Center users should contact their TSR or the Service Desk at (210) 567-7777 option 1 for assistance on the various options.  UTHSCSA has a public FTP site available--click here for instructions.
  • My message is less than 30 MB, so why am I getting an "message size exceeds maximum size " error?
    The 30 MB message size limit we advertise is the actual total size of the entire message during transmission.  Due to the encoding that’s required to transmit attachments via Internet e-mail, the size of an attachment as it is being sent through the e-mail system can be up to 150% of its original size.  This is because the e-mail system must add internal headers and footers for each attachment and each attachment must be converted from an 8-bit format to a 7-bit format.  This, unfortunately, is required by the underlying e-mail protocols that were designed for the transmission of text only, not for binary files.  Internally within our Exchange e-mail system, messages are transported in 8-bit binary format, so that conversion isn’t required and the 30 MB limit is fairly literal.  But messages to external recipients must be converted to the 7-bit format that is the standard for Internet e-mail transport (SMTP); this results in a de facto size limit of 20-22 MB per message.

    Another possible issue is that the recipient's e-mail system has a lower size limit than ours.  For example, if your message is 20 MB and the recipient's e-mail system has a 15 MB size limit, your message will make it through our system but will be rejected by the recipient's e-mail system.

* Internal-to-internal messages are those that go from one UT Health Science Center recipient to another.

Questions or problems?