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Exchange 2019 Mail Flow Issues: Emails Stuck in Queue & Outlook 2019 Connectivity Delays

Anonymous
2025-03-15T04:41:12+00:00

Hey,

I'm running an Exchange 2019 DAG and have been experiencing intermittent mail flow issues where emails get stuck in the queue, mainly on MBX2. Messages stay in Retry state with errors like 451 4.4.0 / 421 4.3.2 Service Not Available, and Shadow Redundancy Manager keeps waiting for acknowledgments, preventing email delivery. Transport logs show repeated retries before eventual failures. Restarting the Transport service temporarily restores mail flow, but the issue keeps returning. I've checked DNS, SMTP connectivity, and the transport database—everything looks fine. Shadow Redundancy is enabled on MBX2, and there are no major resource constraints (CPU, RAM, Disk). Other DAG members are working normally. Additionally, users on Outlook 2019 report delays in email delivery and occasional "Trying to connect" status, which could be related. Could this be a transport database issue or a DAG sync problem? Has anyone seen Shadow Redundancy causing mail queues to stall like this? Would clearing the queue or temporarily disabling shadow redundancy help identify the root cause?

Any insights would be appreciated!

Outlook | Windows | Classic Outlook for Windows | For home

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Anonymous
2025-03-15T05:43:36+00:00

Hello Default username02743

Thank you for posting in the Microsoft Community.

Based on the details you've shared, it seems you're experiencing mail flow issues in your Exchange Server 2019 DAG, with messages getting stuck in the queue on MBX2.

However, I'd like to sincerely point out that since you've posted this in the Outlook / Windows / Classic Outlook for / For business, community members here primarily focus on Microsoft 365 Business Exchange Online, Outlook and mail flow issues.

Since your concern is related to an on-premises Exchange Server setup, I recommend posting your question in the Exchange Server - Microsoft Q&A Community: Exchange Server - Microsoft Q&A

Experts in that forum specialize in Exchange Server deployments and Active Directory schema extensions, so you'll likely receive more precise guidance.

I apologize for redirecting you to another community, but this will ensure faster and more effective assistance from experts in Exchange Server.

Thank you for your understanding and cooperation. Have a great day!

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