Fix Exchange Hybrid Mail Flow

I’m running an Exchange hybrid setup (Exchange Online with an on-prem 2019 mailbox server) and messages sent from the cloud aren’t reaching mailboxes on-premises. Mail flow the other way around is fine, so the fault lies somewhere in the cloud-to-on-premises path.

I’ve already pulled message traces and transport logs; they show several repeated errors but I’d like a fresh set of eyes to confirm the root cause, validate the current Hybrid Configuration Wizard settings, and correct whatever is blocking delivery.

What I need from you:
• Review the existing hybrid connectors, accepted domains, send/receive connectors, and any related transport rules.
• Analyse the trace and protocol log snippets I’ll provide, identify the exact failure point, and explain it plainly.
• Apply or guide me through the corrective steps—whether that’s certificate re-binding, connector tweaks, firewall or smart-host changes—until test messages from Exchange Online land successfully in an on-prem mailbox.
• Supply a short hand-over note summarising what was fixed so I can document it for future audits.

I’ll give you remote screen-share access (or run commands you specify) in PowerShell, the Exchange Admin Center, and the Office 365 portal. The engagement is complete once a cloud user can send to an on-premises user without delay or NDR, and the fix survives a second test after 24 hours.