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Outlook takes 40 seconds stuck at "trying to connect"

Riddle me this batman. I have an isolated lab environment which doesn't have internet access, but does have Exchange, Domain Controllers, and client machines running Outlook. When opening Outlook in this isolated environment, it takes about 40 seconds for Outlook to find and connect to Exchange. If I jump into the lab firewall and give said client internet access, this problem goes away. Outlook client connects to isolated Exchange server instantly.

Exchange 2016 on-prem and Windows 10. Outlook version 2013.

Regards,
Adam Tyler

office-exchange-server-administrationoffice-outlook-itpro
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AndyDavid answered AdamTyler-3751 commented

Fire up fiddler on the workstation and see what the client is trying to connect to

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Thanks for the tip. Here is the output from Fiddler while Outlook is hung "trying to connect".

143068-image.png


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image.png (29.5 KiB)

Sure enough. Once I whitelisted the IP that this resolves to, Outlook fires right up in dev with all other traffic blocked. Unfortunately the list of IPs associated with this FQDN seem to rotate depending on what public DNS server you query. So something you'd have to look every time.

Regards,
Adam Tyler

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AdamTyler-3751 answered

Sure enough. Once I whitelisted the IP that this resolves to, Outlook fires right up in dev with all other traffic blocked. Unfortunately the list of IPs associated with this FQDN seem to rotate depending on what public DNS server you query. So something you'd have to look every time.

Regards,
Adam Tyler

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