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 don’t 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 it’s 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-customers’ servers.
If you have a proper SPF, and the IP isn’t 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: “no need to reinvent the wheel” and “people all have accounts with Gmail and others”.
Yeah, for both of them. Except that:
1/ you don’t 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 (and… 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 ‘control from nextcloud’ mean? I don’t think it more should be in scope than a shared user / group base. And that’s what Ldap is there for: Ldap would ‘control’ 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 Exchange…
Considering that managing email servers today isn’t 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 miserable…
Don’t get into fights you have zero chances to win…
Sorry to say it, but It’s a so bad argument!
You consider that fight is lost because you don’t have the leadership. I managed email server for many years (Postfix then Exchange), it’s not so complicated. Exchange is much more unnecessarily complicated. Especially because there’s very little useful log and most of the time, when there is a problem the answer you read everywhere is “restart the services”!
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.
Let’s 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 there’s Talk into NextCloud? It can be sometimes complicated to configure it and make it work perfectly while installing JitsiMeet takes… 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 don’t wand SAP, we want GSuite or Office365. Perhaps Microsoft365 will win the “war” because Because the Azure/Windows couple will be very difficult to dethrone. But it is possible to start by unbolting some statues (I don’t know if my translation is understandable :D)
That’s 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 one…
And? How much market share does Postfix have?
Probably less than Linux has desktop market share…
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/Skype… They got the lion share of the new pie… And they might carve out a niche for themselves as the dust settles but not by winning battles…
They did not take users away neither from the big guys not BBB/Jitsi…
Thank you. The first and only sober thought…
There is something to be said about established markets…
Steve Jobs admitted defeat in the desktop/office market decades ago.
Probably the smartest thing he did…
Don Quixote’s fight with the windmills is good in literature, only…
0/ “How much market share does Postfix have?” 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. Let’s do like this. Sincerely, I don’t care, my company use (I’m 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 “critical thinking” and use Microsoft365…
Look up the definition of “oxymoron”…
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this is getting a bit too OT here, I think… 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 –
Something here I think you should read – https://nextcloud.com/contribute/code-of-conduct/
I’ve got a super simple server setup and control panel system that could be turned into a NC app. It’s 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 it’s 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 parts…
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I don’t know, i use the nextcloud calendar apps because the invitation system works better than the SOGo one.