On what kind of provider you are (Google, AWS, 1&1)?
On a Dedicated server, VPS, Private Cloud, shared server ?
Maybe OVH VPS IP are badly seen from Microsoft
I donāt use VPSs personally. The IP Addresses of the mail servers I maintain belong to various German ISPs or Datacenters.
I made the same experience. Used a clean Ip on a Dutch VPS that is not listed in any blacklist. I got a 10/10 score with all required records correct. All the big providers treated my mail as spam while smaller ones accepted it. Lots of hassle! So now I decided to use an external provider for sending mails 
I think itās more likely that the big providers are distrusting known VPS IPs after having seen who knows how many misconfigured or spam-laden servers come and go from them, rather than them targeting competitors or non-customersā servers.
If you have a proper SPF, and the IP isnāt listed on a public blacklist/extortion racket, then it should be fine.
You can check for listings here: https://mxtoolbox.com/blacklists.aspx
Yes, that might be the case
Unfortunately not. Everything ok in my case, not blacklisted, 10/10 points from another tool, so the above mentioned mistrust to VPS IPs seems to make the difference
All the disagreeing answers I read always have the same arguments: āno need to reinvent the wheelā and āpeople all have accounts with Gmail and othersā.
Yeah, for both of them. Except that:
1/ you donāt have to reinvent the wheel. Postfix works very well, Spamassassin also, OpenDKIM, OpenDMARC also. And for the admin, there are probably elements to take everywhere (for example webmin)
2/ I studied at length the possibility of having NextCloud as an alternative to Microsoft 365 in my company (250 users). If there is a major problem on NextCloud (besides the UX), it is the lack of mail server. Even if the mail server is elsewhere (and this is preferable), you should at least be able to control it from NextCloud.
My opinion is that if you want to continue to target NextCloud to the individual, ok, just add a decent mail client (andā¦ no need to reinvent the wheel :)). But If you want the tool to one day be adopted by companies, so be an alternative to Microsoft365, Gsuite and Zoho, you have to go further.
what would ācontrol from nextcloudā mean? I donāt think it more should be in scope than a shared user / group base. And thatās what Ldap is there for: Ldap would ācontrolā authorization&accounting for both of them.
The on-premises email server market - what Nextcloud would be competing against if offering an email server - is dominated by Microsoft Exchangeā¦
Considering that managing email servers today isnāt exactly walk in the park, plus, you as small fish are at the whim of the downstream big fish players, the ROI of such a move would be miserableā¦
Donāt get into fights you have zero chances to winā¦
Sorry to say it, but Itās a so bad argument!
You consider that fight is lost because you donāt have the leadership. I managed email server for many years (Postfix then Exchange), itās not so complicated. Exchange is much more unnecessarily complicated. Especially because thereās very little useful log and most of the time, when there is a problem the answer you read everywhere is ārestart the servicesā!
If you use Postfix and configuration tools from webmin and others, problems will be rare. And, just like with the rest of NextCloud, when tough problems occur, no ordinary user is able to solve them.
Letās take an analogy: Zoom is domining the visio market with Teams, and for SOHO, Zoom, WhatsApp and Skype. Why excellent tool like BigBlueButton try to exist? And, most of all, Why thereās Talk into NextCloud? It can be sometimes complicated to configure it and make it work perfectly while installing JitsiMeet takesā¦ 0 second.
But also : Office is dominating office suites, Windows is dominating OS, Samsung dominated the smartphone market, Altavista totally dominated search engines, etc.
An (theorically) excellent tool like the MS fluid framework (open source :)) show that all this office tools (edition and collaborative tools, communication tools) are made to work together.
We donāt wand SAP, we want GSuite or Office365. Perhaps Microsoft365 will win the āwarā because Because the Azure/Windows couple will be very difficult to dethrone. But it is possible to start by unbolting some statues (I donāt know if my translation is understandable :D)
Thatās my point.
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If someone wants to step up and build a mailserver as an app, then they could take the Onlyoffice community document server and the built-in Collabora CODE server as a reference on how to integrate huge software packages into Nextcloud:
https://apps.nextcloud.com/apps/richdocumentscode
https://apps.nextcloud.com/apps/documentserver_community
I prefer to make a bad argument as opposed to a lame oneā¦
And? How much market share does Postfix have?
Probably less than Linux has desktop market shareā¦
No, it does not.
Zoom is the most popular video conferencing tool in the new market created by COVID-19.
They did not dethrone WebEX/Teams/Meet/Skypeā¦ They got the lion share of the new pieā¦ And they might carve out a niche for themselves as the dust settles but not by winning battlesā¦
They did not take users away neither from the big guys not BBB/Jitsiā¦
Thank you. The first and only sober thoughtā¦
There is something to be said about established marketsā¦
Steve Jobs admitted defeat in the desktop/office market decades ago.
Probably the smartest thing he didā¦
Don Quixoteās fight with the windmills is good in literature, onlyā¦
0/ āHow much market share does Postfix have?ā Humour? How much market share does NextCloud have? Lame you said ? 
1/ Zoom pulled all the other solutions up. Teams had to adapt very quickly.
2/ Apple has greatly multiplied its market share since its near-death experience when even clones were allowed.
But no need to argue anymore. You right. Totally. Letās do like this. Sincerely, I donāt care, my company use (Iām the CIO) Microsoft365 and NextCloud will wisely continue to be a niche tool for a few geeks thanks to a total lack of critical thinking like yours.
Goodbye, sweet dreams.
So, you are the CIO of a company, have abundance of ācritical thinkingā and use Microsoft365ā¦
Look up the definition of āoxymoronāā¦
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this is getting a bit too OT here, I thinkā¦ the subject was something different from where you guys just ended.
maybe you wanna go on discussion via direct messages? 
Something here I think you should read ā
Iāve got a super simple server setup and control panel system that could be turned into a NC app. Itās on my todo list but the system needs work, better error reporting and docs. The server setup is pure bash and the simple control panel is plain PHP without dependencies (no composer, python, ruby, nodejs, whatever required). If a few other folks tried to use it and gave me bug reports and pull requests it could be whipped into more general shape fairly quickly. Some pointers about how to turn the PHP part into a NC app would speed up that side of things too. ATM it only runs on Ubuntu servers. I use it for real with up to 500 web/mail/dns clients so itās more than 90% usable right now.
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Any idea to import from sogo in mailcow the Calendar into Nextcloud as an subscription?
I also use mailcow as my mailserver.
@soliloque probably you are not aware that you are NOT talking with Nextcloud CIO or support. This is a forum in which mainly the users write.
If you are the CIO of a company and you want to ask Nextcloud GmbH if they would support a mailserver, please do.
I think you can look at the info to contact them here.
Maybe you could find an arrangement with them good for both the involved partsā¦
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I donāt know, i use the nextcloud calendar apps because the invitation system works better than the SOGo one.