It sounds like you have a rule that blocks these digests or a rule that triggers something within the digests and blocks them. We do not recommend using on box quarantine, however, you can do that. Best thing to do is use off box quarantine such as MQM and load it on a 2003 or 2007 Exchange server. MQM 6.0 software is free. Once you set that up, when something will get quarantined, it will get pushed to the MQM and you shouldn't have a problem.
With regards to your issue, if you choose not to quarantine off box, you need to see what policy gets triggered and remove it or modify it. You can also call into support and with that, we'll need you to generate a MER report in case this needs to be escalated. MQM should be available to you when you sign into McAfee with your grantID. If you run into problems finding it, please call into support so we can help you with that.