ChatGPT Writer is a general-purpose AI tool that can draft text, check grammar, rewrite text, change tone, translate text and more. It can be used directly in Gmail and supports many languages. It's a mature tool with strong security protections. It's available in several pricing levels, including a free tier, so you can use it for whatever you need.
Another good option is WriteMail.ai, which analyzes your email in real time and suggests improvements to clarity, concision and tone. It works with multiple languages and has several pricing tiers. People have reported a big time savings and better emails, so it's a good option for personal and professional use.
If you're looking for something more collaborative and feature rich, Superhuman is worth a look. It's got a split inbox, AI writing, follow-up reminders and integration with both Gmail and Outlook. Superhuman is geared for high-performing teams trying to free up time and manage their inboxes better.