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I've been hosting full featured email servers for half a dozen organizations, mainly non-profits and healthcare institutions, for a few years. It's used internally & externally by individuals, for their daily work.
I successfully implemented all email best practices, from strong authentication to SPF, DKIM, DMARC & enrolled into all postmaster tools (like Microsoft SNDS, Google Postmaster, etc). Every email is scanned against ClamAV (antivirus) and Rspamd (antispam), also each server has its own dedicated IPv4 and IPv6 addresses.
Since the beginning of this year, Microsoft started to reject emails sent from these servers. It took me almost 3 months between January & March to fix this issue with them (which was a false positive in their systems), and now they are bouncing my client's emails again.
I sincerely don't know what to do, I'm 100% sure no spam is originating from our side. My clients (I don't blame them) are lost and don't understand why their emails aren't delivered. I explained many times that they are being blocked by Microsoft services and that everything is working as expected on our end. Still, even workaround like relaying sending through Sendgrid don't work, their IP addresses are blacklisted from Microsoft as well.
I'm feeling bad about this, even more, helpless with this issue coming back. I've ran emails servers for years and never had issues with any other email service providers (Google, Yahoo, etc).
What do you think should I do? Propose my clients to move to Office365? I believe it's the only way to fix their deliverability issues with Microsoft... In the meantime moral support is greatly appreciated.