Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Dr.Explain - Dr.Explain software is a help file authoring tool.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Document360 - Self-service knowledge base software focused on SaaS Products & Enterprise Software Projects.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
WinCHM - Creating CHM file, Make CHM file, Web Help. WinCHM Pro - Help authoring software (CHM, WebHelp, DOC, PDF) . With this powerful tool you can not only create chm file from scratch, but also convert a set of existing HTML files to a chm file.