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updated 11/15/2012 11:20:55 AM ET 2012-11-15T16:20:55

From vendors to clients to marketing outreach and beyond, countless emails fly in and out of a small business each day. And with sensitive information sometimes at the heart of these digital messages, entrepreneurs rightly want their email application to be as secure as possible.

While web mail giants like Google, AOL and Yahoo already offer a high level of security, business owners still should consider enterprise-level controls, advanced privacy features and complex encryption to protect their information. Here's a look at five simple tools that can help keep sensitive material such as bank account data, human resources information and health compliance forms safe over email:

1. Microsoft Office Trust Center
Microsoft sees security as a competitive edge over Google and has built new security features into Microsoft Outlook. For example, Office 365 -- which is the online version of Microsoft apps like Word -- has built in an "Office 365 Trust Center." Included in the cost for Office 365 business plans, which start at $6 per month, it includes encrypted messaging and the means to capture legally-binding online signatures. For shops already using Office 365, these tools come built in.

Keep in mind, additional servers and other software may be needed to run some security services on traditional, PC-based Microsoft products.

Related: How to Avoid One of the Biggest Email Hacking Threats

2. Hushmail Business
Vancouver, Canada-based Hushmail offers a painless way to send secure, encrypted emails. Accounts start at no cost, and the interface works similarly to many familiar email tools such as Gmail or Hotmail. The encryption technologies are complex, but the service is not. It simply encrypts every message sent so that only the recipient can read it.

But unless firms choose to migrate the entire email service to it, Hushmail requires additional email identities -- each requiring new passwords and the need to remind clients to use it. Also, Hushmail bills for additional features. So costs can mount for services such as document protection or desktop access. The basic service is free. Paid plans, which include unlimited email aliases and business domain names, start at $1.99 per user, per month.

3. Sendinc
Though aimed mostly at consumers, Dallas-based Sendinc offers a quick web-based way to jump into offering secure emails for firms. Sendinc provides a business-friendly, stripped-down interface and there's a handy plug-in for Microsoft Outlook that makes business migration relatively painless.

The basic service is free. Paid plans -- which support larger emails, built-in expiration dates and multiple accounts -- start at $5 per user, per month.

Related: 7 Ways to Make Gmail Faster

4. Enlocked
Palo Alto, Calif.-based Enlocked is a web-based, free plugin that allows users to easily send secure emails via their smartphone, email client or web browser. Sign up takes roughly 30 seconds and the service includes mobile device support. The tool can be particularly apt for businesses that only occasionally require advanced features such as email encryption. And it was easy to add other features like expiration dates to messages.

It's worth noting that Enlocked manages much of the back-end security technology on its own equipment, which some people, including Seattle-based software engineer and author Adam Monsen, say makes email less secure. According to Enlocked, its system doesn't necessarily compromise security but offers an improved experience for customers.

5. RPost Office
Los Angeles-based RPost takes a distinctive approach to secure messaging. It offers encryption as part of a registered email and eSignature product. The goal is to create a legally binding, healthcare-industry compliant message platform.

RPost provides detailed proof of delivery that is more comprehensive than typical "read receipts." Users receive timestamps, a list of attachments that were shipped with an email and authentication certificates that verify who viewed and interacted with a document, proving an email was delivered and viewed. The company also has a built-in signature capture feature that allows businesses to send and receive legally binding documents.

RPost Office costs $14.99 per user, per month for up to 100 emails. Those fees can add up fast. But it can be a significant asset for industries such as law enforcement or healthcare that rely on documents.

Related: How Not to Be a Spammer When Marketing Your Business

Copyright © 2013 Entrepreneur.com, Inc.

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