An email system with a 20MB size limit rejects your email with a 20MB file attached. Email users are often puzzled by this. When your email is rejected due to its size, it is not the attachment size alone that’s causing the issue. Each message contains some overhead, such as the encoding to convert the attachment to text when sent, then back to its original form when received.
The MIME standard introduced by RFC2045 in 1996 provided a framework to encapsulate a message and all its attachments in a single multipart message. This standard uses base64 encoding to convert binary into 7-bit ASCII. The base64 encoding adds up to 30% overhead. Meaning, a 20MB document on disk exceeds a 25MB file attachment limit. So that, the largest email attachment size allowed here at the Institute is more like 15MB. For files under 15MB, you are guaranteed that it will go through the IAS email systems without problems.
In addition to this information, there is a new limit added to our top level mail servers called a file expansion limit. That is 60MB. This means that when the mail message is received by the server, which must be under 17MB and it expands a compressed file (like a zip or tar), the size of any individual file uncompressed must be under 60MB. This limit will aid in the efficiency of email scanning to provide protection from spam and virus infections. Imagine if a zip or tar had many files that expanded to 100MB each; this would bog down the scanning server.
Use your public web directory to share large files instead of emailing them especially for files over 15MB. Information on how to use or access your personal web directory may be found here