The 400 is splitting your attachment (don’t ask what that means or why IBM does that) Just use CHGPOPA command to set your message split size to *NOMAX
IBM does it for the same reason most other e-mail servers do something similar; the difference is that others generally have larger default limits. Most often, e-mail servers have a maximum size limit. Under OS/400 or i5/OS, rather than just blocking the transmission, the attached file is automatically sent in segments. A client that can reassemble the parts is needed.
Technically, SMTP e-mail is not intended to be a ‘file transport’ mechanism. That’s what FTP, etc., is for.
E-mail servers shouldn’t be taxed by trying to process large files through a protocol that is explicitly designed to handle relatively small text messages. It’s bad enough that spam clogs e-mail servers. Large files put a definite drag on them. (That’s all from the perspective of the purpose of SMTP e-mail. The reality is something else entirely.)