How can I exchange large file?

As email is such a handy tool, people are very often tempted to use it to send almost everything, say computer files of graphical images, word-processor documents, audio/video data, or even operating systems which have size as large as several to tens of megabytes!

However, the result can be DISASTROUS because of the following reasons:

  1. It may cause unnecessary loading on the network traffic and, in the most undesirable case, even the whole email system will be affected.
  2. The sender's email client may encounter problems such as email connection time-out when either the server or the client is very busy. The chance for this to happen is high when sending large mail. The client will retry to send the same message at regular intervals. This will impose extra loading on the mail server.
  3. The condition is obvious when the receiving side is remote or unstable such as poor line quality.
  4. It may become worse when some of the messages are addressing external parties whose mail servers have limited the receiving mail size (most external servers especially ISP will limit the email size to 10 MB). The mail server will fail to deliver and the mail will bounce back to the senders.

You are recommended to avoid sending single large file which is bigger than 20 MB via email. However, if you need to share large files with others, you should adopt the following approaches:

  1. Large files are most efficiently exchanged through Web service. Your mail content can include the URL referenced documents instead of file attachments. You can save your files in either one of the following servers:
  2. Use the FTP (File Transfer Protocol) which is a common tool for file transfer over the Internet. Since the FTP can run on many computer platforms, it is the ideal choice if you and your recipient have different computing environments. Since the FTP is an interactive tool, you can see what is going on during the file transfer process. If interruption occurs, the problem causing it can be known and re-transfer can be initiated at once, if desired.

If for some reasons you need to send your big files through email, please refer to the following hints to minimize the size of your big files before sending them:

  1. Please DO NOT send files with unnecessary details when you share an image with others. In order to minimize your image file size, you are recommended to refer to the following steps to create it:
    • You should save your image files in GIF format (For 256 colors) if they are not supposed to have high colors.
    • For high colors image files such as picture or photograph, you need to save them in JPEG format. However, a 800 X 600 resolution is enough for viewing purpose in most cases as higher resolution will create very large file size.
  2. Some tools are available (freezip or winzip) for you to compress your files (e.g. Microsoft word, Excel, PowerPoint and Programs) before sending them. You can access FreeZip from the Dept'al Applications Menu of the CityU Intranet. Please note that this approach is not suitable for files already in compressed format such as JPEG.

  3. Split the original file into several smaller files and send them in separate email. This method is most suitable for sending material like word document that can be broken down and put together, say based on page number, chapter, section, or the like.

If the Internet connect is unstable at the receiving side, you may need to back up or copy the files on to CD-ROMs or DVDs and send them by post. Remember to pack them properly so as to avoid bending or damage during the mail.

Return to University In-house Email Service FAQ page