Last week, Twitter slowly started rolling out a new feature that allows users to receive a direct-message from anyone following them on Twitter. The current setting only allows users to send a direct-message to those who follow them, leaving a potential recipient without the option to send a DM back.
The new feature isn't on by default, as it requires you to specifically enable it via Twitter's Web site.
You'll need to visit your account settings to see if there's a new Messages option under the Content section. If the option isn't present in your account, periodically check back.
By checking the box, you're granting anyone who follows you permission to send you a direct-message, without you having to follow them back.
This is a huge boost for brands who use Twitter as a customer service outlet, by eliminating the need to follow thousands of users. This could potential lead to a renewed influx of spam. Twitter recently disabled the ability to send links in direct-messages until a fix is in place to ensure all links are safe.