Think of email as a postcard that anyone can read...
Technology lawyer Mark A. Berman writes in the New York Law Journal that you should stop thinking about your inbox as a private place:
This applies to attachments as well as the email text, an easy solution to the email security issue is to use GoFileDrop to receive files as they are transmitted directly to your Google Docs account, they do not travel through potentially insecure mail servers.People continue to rely on their belief that the contents of e-mails, like phone calls, are sacrosanct and what is “said” in e-mail communication remains “confidential” to everyone other than the parties to them.However, that expectation of privacy is breaking down by the day. E-mails should more properly be viewed as a “postcard” or a conversation over a speakerphone, both open and available to a passerby to hear or see, than like a private “confidential,” “sealed” letter.
Furthermore the files are transmitted using a secure connection - the same type used when you send your credit card details to make an online purchase.
The bottom line is if you want to ensure the file someone is sending you cannot be read by third parties then get them to use GoFileDrop.