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The following Cisco bug documents that when Exchange is federated with Outlook.com the Outlook user agent changes so that any email based image is blocked as webmail on the ironport.

https://tools.cisco.com/bugsearch/bug/CSCut39521/?reffering_site=dumpcr

Is there any reasonable workaround on the Exchange side to correct this? (other than disabling the connection to O365)

makerofthings7
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I'm not sure it actually has anything to do with Federation. We aren't federated, and after the Mar 10th signature update it began blocking our on-prem Outlook clients from downloading images and RSS feeds. Ironport appears to be filtering on the UserAgent, which does include the text Microsoft Outlook, and then blocking based on the Webmail (Outlook.com) rule. Their workaround was to unblock Webmail/Outlook.com until a signature update is made to Ironport.

guest
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  • what signature update was that? The Exchange AV, OL, or Cisco? – makerofthings7 Mar 18 '15 at 14:21
  • On the Ironport. Specifically AVC. **Cisco IronPort Web Usage Controls - Application Visibility and Control Data: 1.1.0.30-004 (Tue Mar 10 18:28:49 2015)** – guest Mar 18 '15 at 15:13