Fmail is a native application shell for FastMail. For iOS and Android there are native clients, but not for macOS.

The web interface used by FastMail is excellent. The one thing missing is that the web interface cannot handle email links. So when you click on such a link it is the macOS Mail application that opens. That is not what we want. Enter FMail.

Other features:

• Handles email links from other programs/documents
• Notifies you when new mail arrives
• The Dock icon can show the number of unread mails in the Inbox or in all folders
• Status Bar Menu for easy access (will show you when there are unread mails)
• Activate/deactivate FMail window via configurable shortcut
• Search on a word in all emails via a right mouse click (contextual menu)
• Can run without Dock Icon (as background application)
• When run as background application, you can let the window close when going to another application (but FMail stays open, when you are composing an email)
• Detects when FMail is not the default email reader
• Window bar shows the title of the selected email
• Mouseover shows email links (so you can decide if you want to click a specific link)
• Use a right mouse click to immediately search the selected text in all your emails
• Attachments are downloaded from inside the application
• Define your Downloads folder
• .pdf files and images can be previewed and/or downloaded
• Show Raw Message inside the application and the text is easily copied to the pasteboard
• Gracefully handles Javascript confirmation alerts
• Application specific contextual menus
• FMail uses Safari for handling Print commands*)
• Shortcuts for composing and searching mail
• Can start at login
• Auto dark/light mode in Mojave
• English, French and Dutch localizations
• FMail is a Notarized and Sandboxed application
• Written entirely in Swift
• Is actively maintained (because I use FMail myself)
• Application size less then 10 MB