For years now, I use Fastmail as my email provider
FMail, the free native Mac application for Fastmail users.
Have a look if you do not use this service and you want to know what it can do for you. Over the years it kept improving and now it is very powerful 1).
For iOS and Android there are native clients, but not for macOS. The web interface used by Fastmail is excellent.
The big thing missing for me, is that the web interface cannot react like a native mail application. It can, for example, not handle email links. When you click on such a link it is the macOS Mail application that opens.
That is not what we want.
Enter FMail or FMail2

When you have installed FMail and have adjusted your default email client, an email link will open the FMail client with all provided fields filled.
There are a lot of extra options too.
Downloading is handled inside the application and the files are saved in your Download folder (or to another folder of your choice).
Want to look up a tracking number? One right mouse click away.
If you use Fastmail from your browser switch to FMail. For free. Guaranteed no tracking of your data.
You won't regret.
FMail is a native Mac application, taking little resources on your system and runs natively on Apple Silicon.
Features

Default email client
As FMail is a native macOS email client, it handles email links from other programs/documents.
Be sure to set FMail as default email client.
FMail will notify you when it is not the default email client.

Notifications
Notifies you when new mail arrives. Choose the notification sound and select if you want to see the number of unread emails for the Inbox only or for all folders.

Dock Icon
The Dock icon can show you the number of unread emails. You can set this for the emails in the Inbox or in all relevant folders.

Run in the background
FMail can run as background application with only a Status Bar menu. It is possible to open FMail via a global shortcut key or from the Status Menu.
Handy: In the Preferences you can define that the FMail window closes when it loses focus.

Preferences
You can set FMail to your liking in the Preferences (⌘,).
Let FMail work the way you want.
Set FMail to listen for keyboard shortcuts FastMail does not have:
Use the up/down arrow keys to loop over your emails, contacts and more. With the left/right arrow keys you can switch fast between email - contacts - calendar.

Secure
Apple wants to keep all software on your computer safe.
Therefore Apple wants that applications are Notarized and Sandboxed. So FMail is notarized and sandboxed 2).
FMail is written in Swift, a safe modern language.
Another benefit from not running Fastmail in your browser is that FMail protects you from cross-website trackers.

- Hide FMail from the dock
- Select your Download folder
- Use the Arrow Keys to navigate the mails and to switch between Mail, Contacts and Calendar
- The Status Menu shows the number of unread emails
- FMail can change between dark- and light appearance when macOS does 3)
- Use a right mouse click to immediately search the selected text in all your emails
- Mouseover shows email links (so you can decide if you want to click a specific link)
- Attachments are downloaded from inside the application
- Pdf files and images can be previewed and/or downloaded
- Show Raw Message inside the application (the text is easily copied to the pasteboard)
- Gracefully handles Javascript confirmation alerts
- Window titlebar color adjusts to the chosen Fastmail theme
- Supports Hook linking via standard (RFC-5322) email ID's
- Localized for English, French and Dutch
- Is actively maintained (because I use FMail myself)
- The application size is less then 10 MB and uses little memory
- Automatic updates (if you want)
- Runs natively on Apple Silicon

Free Download
Open this page on your Mac to download FMail
FMail runs on macOS 11.0 Big Sur and higher
and is notarized by Apple.
~
Maybe FMail2 better fits your needs?
If you find FMail useful you could
Disclaimer
If Fastmail decides to change the underlying code, there is no guarantee I can keep FMail alive.
The program is free to use and without any warranty.
1) I have thought of recreating Fastmail as a normal macOS application, like macOS Mail is. The big advantage would be that I am no longer dependent of the way Fastmail is coded for the browser; if the Fastmail developers make a substantial change in that code, FMail could stop working.
In theory rebuilding is possible for the email part by using the JMAP standard.
This would mean a huge amount of work.
Would that result in a better experience than what the web interface of Fastmail offers? I doubt it.
Another non trivial thing is that at the moment the JMAP standard only supports the email part. No option (yet) for the Contacts and Calendar parts.
So there is very little chance I am going to rebuild FMail as an application that is more then a shell around the Fastmail web pages.
2) FMail cannot be distributed via the Mac App Store because Apple refuses applications that offer subscriptions other then from Apple (see Fastmail Settings → Billing & Plan).
3) With the current version of Fastmail it is Fastmail that can adjust its theme to the macOS theme (light or dark). If you want auto adjust, you have to select this in the Fastmail Settings → Customise → Preferences → Switch light/dark automatically to match system (so not in the FMail Settings).