Google has given a preview of what to expect at a congressional hearing next week when the privacy of internet users takes center stage.

In a letter to three U.S. senators that became public on Thursday, Google defended how and when it lets app developers at other companies scan and share the contents of Gmail accounts, saying that it continuously vets the developers who have that level of access.

Gmail users can opt in to letting a non-Google app have access to their email in a process that Google told lawmakers is transparent. An app must obtain consent and have a privacy policy that goes into detail on how the data will be used, Susan Molinari, a Google vice president for public policy, wrote in the letter.

Digital privacy is the focus of a Senate Commerce Committee hearing scheduled for Wednesday. Google’s chief privacy officer is among the announced witnesses.

The company, owner of the world’s No. 1 search engine, already angered some U.S. lawmakers this month when it declined to send a senior executive to a Senate Intelligence Committee hearing alongside executives from Facebook and Twitter.

Google’s letter about Gmail was first reported by the Wall Street Journal. The newspaper reported in July that Google had allowed hundreds of outside app developers to scan Gmail inboxes — sometimes reading people’s emails — and that Google did little to police them.

Google responded in a blog post at the time that it reviewed apps to make sure they met Google’s policies and suspended them when they do not.

About 1.4 billion people use Gmail, making it among the most popular internet applications worldwide. The company offers both a free version and a paid version, known as G Suite, used by some businesses.

Google said last year that it would stop its own scanning of Gmail messages in order to sell targeted ads.

In May, though, Google told NBC News that it does still collect data from the email of Gmail users in “narrow use cases” such as to personalize Google search results, to detect spam and to build artificial intelligence tools.

Other witnesses scheduled to testify next week include executives from AT&T, Amazon, Twitter, Apple and Charter Communications.

In April, Facebook CEO Mark Zuckerberg testified over two days before Congress, facing questions about the social network’s relationship with outside app developers and how Facebook data ended up in the hands of political consultancy Cambridge Analytica.