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blocklist MS limit exchange spam Understanding the Safe and Blocked Senders lists
How many addresses can the safe and blocked lists hold? Approximately 2000 total. This is because the total size of all lists combined cannot exceed 512K. Once the 512K limit is reached, Outlook alerts you and additional addresses aren't saved until you remove some from the list.

For example, if you have 1800 addresses on the safe list, you can only block approximately 200 addresses. If the addresses are long, your list will hold fewer addresses; if they're short, you'll be able to have more.

Importing a list of blocked addresses from an older version of Outlook is not recommended, in part because these lists often are larger than 512K and Outlook crashes when trying to import them, but also because spammers change addresses so often that lists are outdated within a few months.

Microsoft imposed the limit to improve performance, as the lists are stored as a hidden property in the Inbox and duplicated to the local registry. Each time the users synchronize with the Exchange mailbox store, the lists are downloaded. The limit can be adjusted on Exchange server, by adding a new registry key value to the following registry key:

Add a new
DWORD value of
Type Max Extended Rule Size and edit the Value Data, entering the maximum size (in bytes) you want to allow.

If Outlook's Junk email filter doesn't offer enough options, misses too much spam, or you need better Safe and Blocked lists, try one of the many excellent third party anti-spam filters available.

For a list of third party tools, visit slipstick.com

For Exchange server junk mail filtering tools, visit slipstick.com