Thank you all for participation and suggestions, but the problem was at frontline FIREWALL. I would probably not notice, but I did FW upgrade too and as a post-upgrade went through FW logs.
I found a lot of "One way traffic" alerts, which made me thinking.
And I found the typo in NAT REFLECTION rule, which mapped outgoing Zimbra traffic to different public IP rather than to ZCS dedicated public IP. So traffic for all ports, except for 25, went out through the wrong public IP, rendering numerous errors on clients.
- Zimbra Collaboration 8.6 Patch 9 now available (includes fix for CVE-2017-8802). Read the announcement.
- Zimbra Collaboration 8.8.7 + Zimbra Connector for Outlook 8.8.7 are available.. Read the announcement.
- Are you a Zimbra Developer? You can find some interesting stuff in our Official GitHub: https://github.com/Zimbra and check the Community Projects too: https://github.com/Zimbra-Community/
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