It would seem that Postbox is still available for download:
iOS + MacOS Mail + Thunderbird (linux) with Fastmail as host.
Mimestream for my personal mail, which is on Google Workspace.
Mostly Microsoft Outlook for my work email, which is on Office 365, though sometimes I use Apple Mail for that.
I am tempted. But I am thinking that starting now with a product that has been out of development for several years now is a recipe for some amount of heartache… 
I used to be a Spark user, which I loved. But don’t really send that many private emails so have tested more Apple Mail and Fastmail.
Landed on using the latter, which I feel works very well. Only thing I miss is my iCloud mail in there.
The problem is that my personal account is on Fastmail but my primary client uses MS365 mail so I need a client that supports both.
Why is that a problem?
Because the FastMail client doesn’t support that (I think)
I use mutt, whose website includes the line (presumably from the creator):
In the brief periods when I haven’t used mutt, I felt most at home with MailMate as others have mentioned.
At work it’s Outlook (via OWA). Some flavour of Outlook is mandated by work, and OWA sucks the least.
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I use the Fastmail app because it’s better than most for me, but if you want a single client to handle Fastmail and M365, then my choice would be Spark. It’s the one I used the longest.
MailMaven user here. I love the rules!
I’m using MailMate (latest test release) and I’m very happy with it. It’s the email client I was always missing. It handles multiple accounts, predicate-based mailboxes, >350k emails, IMAP keywords, Markdown formatting, and so much more! I bought my license in 2014 and never looked back.
However, recently I was experimenting a lot with other operating systems and looked for a solution that’s working on macOS and Linux. After a steep learning curve, I’m now also using a combination of neomutt, mbsync, and notmuch for indexing and search. TBH, notmuch is even more faster at searching than MailMate.
Composing HTML emails is still something I’m working on, I’m currently using a pipeline of pandoc with GFM markdown, which can be triggered by a custom keyboard shortcut in neomutt.
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II use both Outlook and Spark. Work is in the Microsoft word, and I using categories. The other accounts are all gmail (primary personal, old personal when I have to provide an address, and scouting mail - 33 years as a volunteer). I try to keep everything viewable in Outlook but it does not allow me to move messages between accounts which I can do with Spark.
I have used Spark Mail as my primary email client on iPhone, iPad and MacOS for a few years now.
With the new features in xOS26 mail, I tried to move back to Apple mail, sadly that failed quickly.
I use Outlook & Outlook via web for work stuff, and do rather hate how it tries to do too much. The interface has so much going on, from
Some reasons why I moved back to Spark:
- archive keyboard shortcut on iPad is unusable, can’t be changed
- the categories are a horrible implementation, the all items is off the screen, you don’t know it is there or has items without scrolling (keyboard shortcuts are also stupidly complex to change categories)
- No clear way (that I know) to change between light and dark mode for emails when reading
- No share sheet, it’s 2026
- Crappy integration into non apple task management apps etc
- Poor search and no smart groups on iOS/iPadOS
What I liked about Apple Mail:
- Email MFA integration (for the many services that won’t move to passcodes or app MFA)
- Better AI summarised notifications
- Native experience across Apple devices
- Better threading view
- Unsubscribe for mailing lists feature
- Better calendar / reminders “smart” features, pick-up items
- better search, including saved searches (a version of smart folders for iPad/iOS
- Swipes that work reliably and are satisfying, mail.app swipe is horrible.
- Snoozing that both moves the message out of the inbox and sets a timer for it to come back, rather than a two step process with mail.app
And overall UX is a important item for me, Spark isn’t perfect, but far better than Mail.App, Airmail, Outlook and a few more that I’ve tried over the years. Mimestream might be interesting, can’t wait for the iOS beta invitation to arrive, as I don’t want to use different clients across iPadOS, iOS and MacOS (I’m a flip flopper).
I have downloaded MailMaven and will use it for a while. I don’t know what I will like or not like but so far it addresses the things that have been bugging me about emClient. 
Thanks for all of the information and responses.
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I get some of the benefits of Apple Mail for Siri picking up calendar events and MFA codes being autofillable by just running it always minimised in parallel with Mimestream.
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@Vincent_Ardern I did this previously also, good tip. Just wish that the MFA worked across devices (as I have a 24x7 macStudio as my server and automation device).
It was perfect until v3 came along. How are you finding it now? I last checked a year after the release and it was still too buggy including multiple crashes on macOS.
I bring all my email accounts into Spark!
Old version of Spark is still available as “Spark Classic” on App Store.
It also receives regular updates