Lavender is a sophisticated AI-powered email coaching tool that works with Gmail, Outlook and HubSpot. Among its features are Email Coach, Personalization Assistant and Lavender Email Score designed to help you write more efficiently and effectively. The tool also offers data-driven coaching and immediate advice, so it's a good all-purpose tool to improve your email writing.
Another option is WriteMail.ai, an AI-based email writing assistant that gives you immediate suggestions for clarity, concision, tone and structure. It works with multiple languages and with several email services. WriteMail.ai also can perform AI editing to catch grammar and spelling errors, so it's a good tool to free up your time and improve your emails.