To prevent abuse of the mail server and encourage better interoperability with other mail servers, we have set the maximum size of email a user can send to 20MB.
Over the Internet a message will often pass through several mail transfer agents to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore need to impose size limits. The result is that while large attachments may succeed internally within a company or organization, they are unreliable when sending across the Internet – and for that reason both sending and receiving systems often arbitrarily limit the size their users are allowed to use.
As a result, while our settings allow for an email attachment up to 20MB, you may not be able to successfully send large attachments to contacts who use other email services with smaller attachment limits. 10MB is generally a safe size for an email (after attachments). Per Wikipedia.org, "In general, 10MB is considered safe for the maximum size of an email.". (http://en.wikipedia.org/wiki/Email_attachment)
Put another way, you may be able to send the 10MB file, but the recipients mail settings may not allow it to pass through their filtering system.
Also, please note that any size limit restriction can be puzzling at times because the MIME encoding adds 35% - 40% overhead – so that a 20MB document on disk may exceeds the 20MB file attachment limit (due to the possibility of added "MIME" overhead of 40%, resulting in an attachment read as 28MB).