Keep buying Exchange™️.
Once Outlook was a mailclient, Pim and a far better Exchange client than Schedule+. Now seems much more Exchange or Microsof365 client. You (WAF, customer, your boss, your coworkers) want the client? Keep buy server and client yearly as Microsoft requests.
I have used and continue to use SOGo, Webtop, and Kopono combined with Nextcloud as exchange/O365 options in environments that I am involved in, however I also support companies that use the Microsoft solutions. I wouldn’t even think of taking them off the Microsoft ecosystem anymore because the reality is MS solutions smoother and much easier to use for the clients. I actually lost a client because I did this once. Kopano is smooth, but not totally open source and the licensing is confusing. SOGo works with Outlook well, but the web interface is heavy and slow. Webtop web interface is much better and integrates with Nextcloud, but ActiveSync doesn’t work right. If any of those 3 would just fix the one area they are week in, they would become a truly good alternative. Since Webtop uses Z-push, I don’t understand why the ActiveSync issues aren’t fairly easy to fix as that is also what Kopano uses.
Hi
I also lost a client due to MS - so what? Win a few lose a few…
I’m glad all those insisting on MS are gone…
I still do have Outlook fans among my clients, but they’re happy with Nextcloud and NethServer…
No ActiveSync, but all their mobiles sync better than with Exchange before (Exchange 2003, was not really usable with mobiles…).
And, I fully agree that MS is smooth, actually, that last Exchange Incident was REALLY slick!
As smooth as can be, even GPO compliant! 
Almost as smooth as Facebook data stealing, even including the initial denials…
PS: The best thing: My “Outlook Fan client”, migrated from SBS2003 in Jan 2000, actually thanked me for saving him a couple of thousand licensing fees on Microsoft, and also for the better security after the last couple of MS “goofs”…
My 2 cents
Andy
Exchange 2003 was connecting only with Windows Mobile (almost not even Windows Phone!).
Anyway… lasts goof were quite… “sizeable”, but were zero-days vulnerabilites into the wild (i won’t talk about Facebook “leak”) but… there were plenty of zero-days vulnerabilities on packages for Linux, but the “stronger point” IMVHO is due to openness of source and bigger crowd who can find issues and problems.
So OpenSource is invulnerable? No. Security is made of choices and processes…
No harsh feelings for people who wants/keep wanting Outlook. Under several points and features is far better than a lot of email clients, when you’re using a lot of the features. And anyway… all people get used to something, and don’t want to change a way to do. In my case, i won’t trade ISO layout keyboard for any reason (may be screwed and cursed the imposer of the ANSI layout!).
Also, for a lot of things now is better (starting from sync between computer and mobile device. Config once, replicate configuration on your mobile device. Nice idea.
I’ll keep using webmail and thunderbird… 
Trying to keep to the main point…because we can all point out all the flaws of whichever config we don’t like… I don’t think it’s unreasonable to ask why we can’t get Webtop syncing well with ActiveSync, that would get us to a point of functionality that would actually rival MS. I’d love to help with that process if I had the coding knowledge to do so, but I don’t. If I were to pursue education that would allow me to do so, which languages would I be looking into?
IMO… Ask to Microsoft.
Ask them to support standard protocols like caldav and cardav “by default”, without third part plugins. Maybe picking login details from the configured email account.
Ask them re-establish a full IMAP compliancy, so i can decide on Outlook which is my Sent, Trash, Spam, Inbox folder, and how the client should behave for deletion (moving, marking, hinding… any setting)
Don’t forget to ask them also to re-establish a full configurability for IMAP, POP3 and SMTP protocol, even for accepting only in the email client, if necessary, self-signed certificate if it’s not verified by compliant/authorized CA (and stopping for ask every single refresh of the mailbox, please!!!). As optional, of course, but without bashing 8 time the head against a wizard that keeps telling “you fool, are you not using Microsoft 365 or Exchange? How miserable you are?”. Just a button “i want to fully configure my email client” without scavenging into Control Panel… Which is going to be MIA from Microsoft soon, into Windows 10.
As default, international and compliant field mapping (including export .CSV proper way, not transposed) for address book, and message prefixes. In Italy, Outlook “wish” to translate Reply with Risposta, Forward with Inoltro. It sucks in so many ways since Schedule+ that i can’t contain my rage about that.
And i should not indulge too much on “bare LF” bad habit. Could lead to meltdown more than RTF Formatting as default or Winmail.dat attachment embedding.
These things should be default and easy to access. Simply, by default RFC Compliant.
I’m personally not asking for client managed sieve rules, currently they don’t work even with plugin for Thunderbird. But at least a real RFC Email client…
Ok, my post seems a quite… loaded with hatred series of requested. They may be not.
If you don’t ask to Outlook to behave like a compliant email client it will work great. Pair it with Exchange and/or Microsoft365 and it can give a lot of satisfaction to the user… I mean… editing rules from client and applied to the server? Wonderful! It’s simple, but it can do a lot of wonderful things even when it’s not opened on a computer.
When you ask some interoperation with other (non Microsoft) software and services… there start the madness. Once there was MAPI library, so at least you could tell to Outlook “do this” (hoping that the version dialect was not so messed up at the next release) and something came out… Nokia Suite, still missing you. You were so nice to talk with all that PIMs…
People wants Microsoft? Ok. Buy Microsoft. Client, and server. Or… stop buying Microsoft, asking them a software that can relate with RFC standards.
Also please don’t forget that MS Outlook, in all Versions, has always been top of the list when it comes to mail based malware…
I do recall the Oracle ad:
“Are you running a Virus Exchange in your server room?”
My 2 cents
Andy
The key has always been to try and wean users off of Outlook! I have had a reasonable degree of success in that over the years and have many using Webtop now.
A reality check for many organisations is pointing out that most of their staff are not using the full potential of Outlook and can replace it with a POP client! The difference being $0 pm for those users. Managers have taken to that comparison and trained their teams on use of Webtop.
I have to agree with @Socs28. The task to move them off of Outlook would be much easier if they have full ActiveSync of Cal and Cards.
Klaus
If the Task is to move users away from Outlook (Fully Agree!) then why would one need ActiveSync?
iOS and Android both do not NEED ActiveSync…
Indeed, all my users sync their mobile devices directly with Nextcloud (Card, Cal and some also files) - and are happy, as it works!
My 2 cents
Andy
Yes,syncing with Davx and Nextcloud android is simple. Its only needed to click in Nextcloud client to davx sync
Yes, Andy it is mostly for IOS/Android email checking. I haven’t looked at Nextcloud sync yet - must have a play.
Klaus
Hi
Nextcloud doesn’t handle Mail (IMAP does that quite OK with the native Apps…) - but CalDAV and CrdDAV both work well for calendars and addressbooks…
Filesync with the App is an additional bonus for Nextcloud…
25+ clients working…
My 2 cents
Andy
Most of my clients and private users using Macs and iPhones. Nethsetver services with Dovecot and SOGo (CalDav/CardDav) works fine* and “out of the box” if you dont use Activesync.
Windows users (if she dont want using the SOGo Webclient) missing Outlook but in most cases she are fine with the “EmClient”.
In “my world” there is no need for Exchange (thanks God).
That’s the most significant value using in combination with NextCloud CardDAV.
I can really suggest em Client. All modules are well integrated…the better Outlook.
There are some small glitches with notification and drag’n drop. But no serious problems.
PS.: and the support is very committed. In the current development version, the notification problem is already fixed.