Many Exchange Server administrators rely on the Intelligent Message Filter (IMF) as an easy and immediately available first line of defense against spam. IMF isn't as tunable as many third-party antispam products (although it can be enhanced with third-party IMF add-ons). That can be a benefit though, because it means much less work. However, you do make a certain tradeoff in functionality.
Exchange IMF, by default, will not scan email messages that are larger than 3 MB. If Exchange IMF runs into a message that's more than 3 MB, it'll log an Event 7515 error in the Application log with the following text:
An error occurred while Microsoft Exchange Intelligent Message Filter attempted to filter a message with ID ####, #### From smtp: #### and Subject ####. This message will not be filtered. The error code is 0x800710f0.
NOTE: The text designated by the hash marks will be different for each message, and should help you figure out which messages are being ignored.
As it turns out, Exchange IMF is designed not to scan messages more than 3 MB. There are several reasons for this:
- Messages 3 MB and over are very rarely spam or unwanted email. The cost involved in sending such large messages on a bulk scale is prohibitive, so most spam tends to be relatively small.
- It takes a certain amount of CPU and memory to scan each message, so not scanning messages larger than 3 MB will save processing power.
- Most organizations have restrictions in place on incoming messages. Anything larger than 2 MB shouldn't be allowed on principle, since it eats both bandwidth and storage.
- While it's entirely possible that a message of that size contains a virus, most email viruses aren't usually that big -- and trapping viruses is not really Exchange IMF's job (and shouldn't be).
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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Related information from SearchExchange.com:
Crash Course: Exchange Intelligent Message Filter
Fast Guide: Exchange Server security essentials
Learning Guide: How to fight spam on Exchange Server
Tip: Troubleshooting Exchange Server error messages
Reference Center: Spam prevention and management tips
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