I have a long conversation with many recipients (I'm organising an event), in amongst the replies I have 3 or 4 mail delivery failed messages, which I'd like to Archive (so that they're not messing up the conversation) but don't want to delete (as I want to refer to them later, to figure out who's addresses are bouncing).
PS I've found a similar question about marking-as-read, but I can't see anything quite like this
How to mark as unread just one message and not the entire conversation in Gmail