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Exchange анти-спам. Миф развеян. - Myth 3: Exchange/Outlook junk mail filtering is one single monolithic entity

 

Myth 3: Exchange/Outlook junk mail filtering is one single monolithic entity.

This is the single largest point of confusion for Exchange administrators, and requires further elaboration due to the complex interaction between Outlook and Exchange junk mail.

Outlook uses its own SmartScreen filter technology separate from Exchange junk mail screening to filter junk mail. While the two sometimes agree Outlook ignores any SCL which Exchange may set on a message and uses its own criteria to determine the "spaminess" of a message. This is the client side filter (It should be noted here that Outlook will honor an SCL of -1. The above only applies to any SCL ratings other than -1.).

Now here is where the confusion begins. The server side filter that Exchange store uses to deliver messages to junk mail has two parts. There is the setting on the mailbox and/or OrganizationConfig object to determine at what SCL value we decide to send a message to the junk e-mail folder. However, for the spam to actually be moved to the junk e-mail folder, a hidden inbox rule has to be enabled. This rule can be enabled in a number of ways. One way is to log into your inbox with Outlook in cached mode. Another is to turn on junk mail filtering in Outlook Web Access (by going to Options and then Junk E-Mail).

Now if you weren't confused already here is the super confusing part- Enabling the rule with the Outlook cached mode way also enables Outlook's client side filtering as well. Turning it on in OWA but not in Outlook (either never logging in with Outlook or going to Tools, Options, Junk E-Mail Options, and clicking the "No Automatic Filtering" radio button in the Outlook client) only turns on the server side Exchange filtering.

In fact while troubleshooting issues with junk mail here in CSS we usually make sure that the client side filter is turned off so we know which filter is not working. Starting with Outlook 2007 sp2 the InfoBar will tell you if Outlook client filtering moved the message or some other filter did the deed (Exchange, rule, 3rd party, etc). See http://support.microsoft.com/kb/968383 for an overview.

If you are not running Outlook 2007 sp2 an alternate way to check if Outlook is moving the message is to fire up MFCMapi, open the mailbox in question, browse to the Junk E-Mail folder, and then highlight the email of interest. Search the lower pane in the "Named Prop Name" tab for a property with the name of "0x859C=34204 = PidLidSpamOriginalFolder, dispidSpamOriginalFolder".

If this property is present then the Outlook client filter moved the message to junk mail.

Your best bet when deploying Outlook clients that will be used exclusively with Exchange is to not install the Outlook SmartScreen filter, as this feature complicates troubleshooting and will not be as effective as the Exchange server based Content Filter Agent.

Speaking of Junk Mail filter behavior we should take a brief stop at Myth 3b.

Myth 3b: Running Set-OWAVirtualDirectory -JunkEmailEnabled $True/False will turn on/off the server side filtering.

This is false. The above cmdlet will only influence the presence of the Junk E-Mail management option in OWA. It has no effect on the server side filter. We get asked all the time if there is a way to programmatically enable junk mail filter within OWA and while there are scripts out there on the Internet that do just that we do not directly support any of them. While these scripts may work now, they are always subject to break with each new RU/SP/version of Exchange. In Exchange 2010 we have a new set of cmdlets, Get-MailboxJunkEmailConfiguration/Set- MailboxJunkEmailConfiguration. These allow us to view the blocked senders and domains in addition to enabling the hidden junk mail rule on a particular mailbox.

Since we are on the topic of Edge servers lets go to myth 4 (ok we weren't talking about Edge servers but that last myth got me really confused and I didn't have a good segue handy)...

 

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