I would disagree, a sensible limit that cannot overwhelm the machine even if a multi threaded server is processing its maximum number of emails, should be enforced.

The correct response to a breach of this limit would be for a reply email to explain the limit and why the email was rejected.

The user then could send multiple emails with their attachments, and the system can be sized to handle e.g. 24 threads processing 64 attachments of a maximum size of e.g. 4096kb

That's how you ensure a system sized to your hardware and ensure maximum throughput for all users.