Sorry, to clarify why this is not just pedantic, the MUA is likely to split up a long list of recipients rather than try to send an arbitrarily long list in one

Sorry, to clarify why this is not just pedantic, the MUA is likely to split up a long list of recipients rather than try to send an arbitrarily long list in one rcpt to command and potentially fail after a very long time processing and sending data. The fact that they don't have to be the same means that the list in the message can be arbitrarily long regardless of how the MUA batches it for the MTA. In that event, it is interruptible. Also, the MTA will batch its own outgoing sends to individual servers (giving them either just one rcpt to or all the recipients whose MX map to that server) and it could be interruptible there as well. The point is that when sending to many many delivery addresses, things get batched at various stages and become more interruptible than if your CC was the master list of how a message were routed.