Mere moments ago, quoth I:

There's not too much problem if the first message id is rewritten (provided both mailman and SES agree on what it is, but that seems hard if mailman is earlier in the traffic chain, as appears to be the case).

Actually, I take that back. Messages might be cross-posted to multiple lists, in which case it's important that these all have the same message id too. While *usually* they would all be Boost lists, that's not necessarily the case, so the only correct choice is to not alter the original message id.