Exchange Server 2013 introduces a change that has an adverse effect on the Outlook junk email filter feature. The end result is that messages excluded from junk email evaluation in earlier versions of Exchange Server and in Office 365 pre-upgrade environments are now evaluated and potentially moved to the Junk E-mail folder.
Background
Outlook and Exchange use a MAPI property (PR_CONTENT_FILTER_SCL) to control the behavior of junk email features in Outlook. Specifically, Exchange can set this property to a value that tells Outlook to exclude the message from client side junk email scanning. This property is referred to commonly as the “SCL” for the message.
Outlook limitation with Exchange Server 2013 and Office 365
With Exchange Server 2013 and Office 365, Outlook does not use the SCL value from the Exchange Server.
The following results apply:
- The MAPI SCL value is ignored, regardless of how it is applied by Exchange
- Outlook sends all mail originating from an SMTP sender to the junk filter for evaluation
- The junk email decision is made by the filter
- If the filter evaluates the email message to be junk, then Outlook moves it to the Junk E-mail folder
Important This does not mean that all email messages always go to the Junk E-mail folder. However, they are not automatically excluded from junk email processing. The junk email filter will still make a decision, and some email messages may appear to work fine, while others go to the Junk E-mail folder. This includes email messages sent from an external trusted domain addresses for which SCL is set to -1 by using an Exchange Transport Rule or the IPAllowList. Also affected is internal email from process servers or custom applications that use SMTP.
Workarounds
- Add the affected sender addresses to the Safe Senders list. Note that if the address includes an accepted domain within your organization, you need to include the full address rather than only the domain (example: alias@internaldomain.com, not just @internaldomain.com).
- Add a mail-enabled contact in Active Directory for the affected internal address. In some configurations, this will allow the sender address to resolve to a friendly name and deliver the message to the Inbox folder as an internal email message.
- Lower the level of junk e-mail protection under Junk E-mail Options, or disable the junk filter by selecting No Automatic Filtering in Outlook.
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While disabling the Outlook junk filter is not a great choice, it ensures important messages are not unexpectedly diverted to the Junk E-mail folder by the Outlook junk filter.
Status and planning
Microsoft is aware of and is currently evaluating this issue for consideration for a fix. As more information becomes available, it will be posted in the following Knowledge Base article:
2885002 Outlook unexpectedly marks messages as junk even if the SCL level is low
Additional resources
If you are unfamiliar with the application and management of SCL -1 as it relates to Exchange Server 2010 and later versions, please see Tom Kern’s excellent post on the Exchange Team Blog:
Accepted Domains, Safe Senders List and You