@vco1 If Postbox had iOS clients I think many many people would be there with bells on. Why they don’t do this is so beyond me…
I’ve been loving Airmail lately, but as others have noted the app’s reliability can be… seasonal. It has been at least a few seasons since I’ve had any issues, though, and it’s gotten slicker and more powerful since. If anyone hasn’t played with it in a few months, it might be worth checking out again.
Have you been able to send later emails using an alias on regular IMAP? Because it never worked for me.
Also, their new Smart Inbox screen is possibly one of the most obscure piece of UI/UX ever made.
MailMate and Mail.app on iOS.
Fastmail with Fmail as the app on my Mac and the official Fastmail apps on iOS.
Mailplane for my Gmail account (mailing lists, forum notifications, etc. All the non-crucial stuff that I don’t want in my personal or work emails. (I use the Gmail app on iOS.)
I realize some people would hate using 3 different mail apps. It works great for me. On the Mac, I use Bumpr to redirect mailto links to the proper app.
I’m a Readdle fan so I’m on Spark. I like the integration it has with their PDF, scanning, and calendar apps.
Shameless plug here for articles I wrote about MailMate and Preside (and the latter for exactly what you said, a superb iOS match for MailMate users):
Email - the topic that will never be put to rest.
I’m currently in what I anticipate will be a decade long transition from having many free email addresses to fewer paid ones. So besides email hosting, I’ve been looking at clients. A recent change in my computing (acquiring an M1 MBP) did change things as I no longer need mail on Windows while my wife used our only Mac at home. Here’s what I’ve settled on, but I’m open to suggestions if anyone has other ideas on what I need.
Mail.app (MacOS) - Mostly for filing mail locally(using rules), for email I don’t need, like coupons or bills. Since the M1 is always running, it continues to apply the rules even when the lid is closed. So I no longer need to rely on my Windows box acting as a mail server. It’s not always immediate, but it works. I wish you could use special characters in filters but that doesn’t seem to work. I also like color coding emails forwarded from older accounts.
Spark (macOS/iOS) - I love the swipe gestures and smart notifications. Signatures is done well, and I can’t believe it took me this long to use templates, that’s been incredibly helpful in my work. But there are no rules or local folders. I don’t use any snooze or send later. I hate when apps create their own folders on my IMAP email or when they point to the wrong one. I like that they collect emails from correspondents, but I also wish they used a true CardDAV.
Postbox (Windows) - I looked at many (when I didn’t have daytime access to a Mac) and settled on Postbox. Best UI for Windows, good local folders and rules. Again, they don’t sync with CardDAV.
Do any of the alternatives to Mail.app (macOS) support the same universal message links that the native app does?
I think this answers your question where MailMate is concerned, unless I’m misunderstanding your meaning.
I was intrigued by that and, after some googling, found that M1 chips “never sleep”. Wow! That almost neglect the need for server side rules.
MailMate is just fantastic. I’m using the latest daily build (which allows for OATH2 authentication necessary for my work account) and have been so happy I gladly paid for a license.
Other apps might have good keyboard shortcut support (but I haven’t seen any as flexible or as powerful as MM).
But more than that, the default “Correspondence” view is a real game changer. It eliminated the need for about 90% of the searching I might do in a more typical mail app.
The “Correspondence” pane is a list showing all your emails to or from the currently highlighted email’s contact. It’s where I go to find out when I last emailed a person, whether they responded, etc.—it’s better than searching a “Sent” folder and a Client folder—and far more reliable than any “threading” view baked into some apps.
And the whole thing is fast.
@macsparky I will be the first one to buy a MailMate Field Guide from you. I can be a beta tester too 
After many years of enduring Airmail’s annoying issues, I am switching over to Spark. Hopefully it will serve me reasonably well.
Mailmate every time. Nothing else comes close
There is a lot of love for MailMate on this forum and I’d be tempted to check it out but without an iOS application, it is a no go for me. I want a consistent app experience across the platforms.
It pairs just fine with Mail.app on iOS. Some prefer Preside as the iOS companion app. But to me Preside feels clunky, complex and ugly in a way that MailMate on Mac does not.
I may give it a try once I order and my new M1 arrives per my earlier post. Does MailMate. Have the ability to create URL links to messages for use in other applications?
Yes, MailMate creates standard message:// links that work in Mail.app. (On macOS, they open in MailMate if it is your default mail app. On iOS they open in Mail.app)
In MailMate I have set the D key to create a new task in Things with a link to the current email message. Very handy.
I did not see Mimestream mentioned. I just downloaded the beta and I am yet to install and test it but the native design looks clean and the concept suits well my Gmail accounts. Anyone tried it ?
Love mimestream 
I’ve been using it since one of the earliest betas. Only any good if you use gmail though!
The fact it picks up my ‘send from’ addresses from gmail and allows me to choose makes it better for me than any other client I’ve used.