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Windows Server 2008 R2 – RTM and SP1 may not respond to a request for a Sender Policy Framework (SPF) TXT Record Correctly if it exceeds 512 bytes

In this scenario if the Sender Policy Framework TXT record exceeds 512 bytes in size then you may find your server anti-spam fails to correctly identify e-mail as either legitimate mail or SPAM, this can result in the two extremes of SPAM mail being delivered to users or legitimate e-mail being rejected as SPAM.

Microsoft do offer a hotfix for DNS Server to prevent this issue KB 2495375 http://support.microsoft.com/kb/2495375