Some browser-based email services provide secure connections while viewing email, but some do not. If the URL doesn't start with https:// (the "s" being the important part), then you are not using a secure connection and an eavesdropper could view everything you send and receive.
Email RouteIn most cases, after clicking the "Send" button your email will travel out on the open Internet passing through various devices before finally ending up at the recipient's server. Has your email been stored or viewed along the way? Who knows? Your guess is as good as ours, but just like sending a postcard, you should probably expect that it could be viewed along the way.
Sent ItemsMany email providers automatically save your sent emails in a separate folder. If you don't actively delete these saved sent emails, then it becomes an easily accessible archive of everything you've sent. If someone gains access to your email account, they could view these items. Additionally, these items are stored on your email provider's servers.
Recipient's InboxOK, so your email has made its way to its recipient's inbox. She has read it and it is now stored with all of her other read emails. Let's say she accidentally left her email account logged in on a shared/public computer. The next person to use that computer now has access to all of her emails including the confidential email you recently sent her. This is just one scenario, but the idea is that you have no control over the email stored in someone else's account.
Send Files Securely is the perfect solution to insecure email attachments! The next time you need to send a file containing confidential information, don't email it. Instead, upload it at https://www.SendFilesSecurely.com and email the link to your secured file.