I recently ran across an issue where mail-enabled public folders will not allow even your Exchange administrators to Manage the Send As permissions on a public folder.  This is typically due to the fact that public folders have been migrated from an older version of Exchange.  For more detail, please find the following article I wrote on how to address this issue.

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http://blog.enowsoftware.com/solutions-engine/bid/156372/Exchange-2010-Public-Folders-Manage-Send-As-Issue