We just migrated a customer from their old IT provider's data center to ours, doing a straightforward lift-and-shift of three virtual machines: an RDS server, a domain controller, and an Exchange 2019 server. After the migration, around 10% of users are experiencing random Windows Security prompts in Outlook, asking for their passwords. Even when they enter the correct credentials, the prompts keep coming back. Curiously, Outlook indicates that Microsoft Exchange is 'Online' and email continues flowing without issues. We've thoroughly checked the client event logs, Exchange logs, and network settings without identifying any problems. The only significant change was relocating the VMs to our data center without altering any internal IP addressing or LAN configuration. Soon, we'll be moving the mailboxes from Exchange On-Prem to Exchange Online using a hybrid setup, and we're curious if this might resolve the issue or if it's due to a persistent Outlook authentication bug. Has anyone encountered similar behavior where Outlook prompts for credentials yet Exchange remains fully functional?
4 Answers
Have you tried rebuilding the Outlook profile on one of the affected machines? Sometimes, that can resolve weird authentication issues like this one.
Just a shot in the dark, but did your autodiscover record point to your Exchange's public IP? It could have changed during the move and not been updated.
Is it device-specific or user-specific? Sometimes the problem can be narrowed down that way.
Okay, you checked some stuff, but what did you actually try? It might be helpful to mention what steps you've already taken.

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