Hundreds of apps are able to scan and share data from the email inboxes of Gmail accounts, Google has revealed.

In a letter to US lawmakers, which was made public on Thursday, Google explained that third-party developers are able to both access and share data from Gmail accounts – though the company said it thoroughly vets any third parties that are granted access, and permission must be given by the user.

The inbox scanning takes place despite Google ending its own controversial email-scanning practice a year ago.

"Developers may share data with third parties so long as they are transparent with the users about how they are using the data," Susan Molinari, Google's head of US public policy, wrote in the letter, which was first seen by the Wall Street Journal.

Google's director of security, trust and privacy Suzanne Frey explained in a blog post in July that Google grants certain permissions to third party apps and services in order to improve the experience for Gmail users.

"We make it possible for applications from other developers to integrate with Gmail – like email clients, trip planners and customer relationship management (CRM) systems – so that you have options around how you access and use your email." Ms Frey wrote.

11 useful Gmail features you didn't know existed Show all 11 1 /11 11 useful Gmail features you didn't know existed 11 useful Gmail features you didn't know existed Labs Labs is a testing ground for a number of experimental Gmail features that aren’t quite ready to be rolled out properly. Most of them are really useful, and it’s worth checking them out and enabling the ones you like by going to Settings and hitting the Labs tab. At the time of writing, one of them shows you how many unread messages you have in the Gmail tab on your browser, and another lets you add handy shortcut buttons. The catch is that they can disappear at any moment. 11 useful Gmail features you didn't know existed Unsend messages You can unsend sent messages with Gmail, and you don’t even need an add-on. Just go to Settings and check the Enable Undo Send option. You can choose between five, 10, 20 and 30 seconds as your cancellation period. After it’s over, there’s no getting that message back. 11 useful Gmail features you didn't know existed Colourful stars The Starred messages feature is already really handy, but you can make it even better. In Settings, scroll down to the Stars section and choose which colour you want to use - you can switch the default golden one for orange, red, purple, blue or green. In fact, you don’t need to use a star at all. Boxes with icons are also available, and you can have lots of them in use at once. It may seem like little more than visual fun, but you can actually make your life easier by using different colours or icons for different types of message, as you can search for each of them individually. 11 useful Gmail features you didn't know existed Shortcuts You can see the entire list of Gmail’s shortcuts by going to Settings and scrolling down to the Keyboard Shortcuts section. Learn More will take you to a page with all of the shortcuts on Android, iOS and the desktop. Or, if you’re already on a computer, just open Gmail and hit Shift and /. 11 useful Gmail features you didn't know existed Hide categories Gmail automatically sorts your messages into distinct categories, including Primary, Social, Promotions, Updates and Forums. However, you can hide the ones you don’t need by going to Settings, hitting the Labels tab and scrolling down to Categories. 11 useful Gmail features you didn't know existed Search properly Don’t be fooled by Gmail’s simple-looking search bar. Click the down arrow just left of the search button to access a load of additional options that make it much easier to sort through your messages. 11 useful Gmail features you didn't know existed Add other accounts If you use Gmail and other email providers, like Yahoo, Exchange and Outlook, you can make your life easier by managing all of your messages from one place. In the app, tap the menu button, add an account by hitting the down arrow and pick the type of account you want to add. 11 useful Gmail features you didn't know existed Smart Replies You can reply to messages quickly by selecting one of three “smart reply” options Gmail offers. It uses machine learning technology to formulate them, to ensure they’re applicable to the message you’re responding to and sound like something you’d write. 11 useful Gmail features you didn't know existed Mute threads Like group conversations on WhatsApp, email threads can get annoying. Fortunately, you can mute them. Open the message, click the More button above it and select Mute. 11 useful Gmail features you didn't know existed Block annoying people You can block users you don’t want to hear from anymore by opening an email you’ve received from them, hitting the down arrow next to the reply button (in the top right-hand corner of the message) and selecting Block. 11 useful Gmail features you didn't know existed Inbox If you’re feeling inquisitive, it’s worth trying out Google’s Inbox (by Gmail) app. It takes a slightly different approach to Gmail, focusing on de-cluttering your inbox.

Ms Frey said that before any non-Google app can access a person's Gmail messages, it goes through a "multi-step review process" that includes assessing the app's privacy policy to ensure that it's a legitimate app.

"Before a non-Google app is able to access your data, we show a permissions screen that clearly shows the types of data the app can access and how it can use that data," Ms Frey said.

"We strongly encourage you to review the permissions screen before granting access to any non-Google application."

The July blog post did not mention that third party developers were able to share Gmail users' data with other third parties.