No one has replied to this yet so I'll test the waters. That seems like a reasonable approach, where you have a slightly more open policy to avoid the quarantine. Exactly what you might want to do depends on your particular environment. Without knowing more what you propose seems reasonable.
Here's a couple of things to think about...
1) Make sure that all DLs really need to be hit from the outside. If you can make a DL internal-only, that solves the problem quickly.
2) Obviously in an educational environment there is going to be quite a bit of concern about missed messages but I have always found the false positive rate from IPAS to be very low. Perhaps some DLs deal with financial information and need extra care but you might find that some DLs can have a tighter set of rules applied.
3) Consider the effect of spam/phishing messages now getting sent to multiple people instead of one so there is a bit of increased risk with DLs.
4) Try to use other data about the messages to help with the decision. Make sure SBRS scores are being used as they go a long way towards an accurate Spam/Ham decision. If you have a place that is a large volume sender of known good emails then consider using a more open policy for them so that you can use a more restrictive policy as a default.
