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