The X.400 is basically an overview of the set of standards. One standard in the series does deal with attachments. SMTP doesn't in and of itself specify a standard for attachments. MIME has emerged as a very common way SMTP handles attachments. X.400 makes it hard to forge, tamper with, or steal messages including non-repudiation.
Checksums help guard against tampering.
SMTP deals with all of these with add-ons, not notably robust. One common approach to this is that email applications such as Eudora and others contain built-in links to Public domain encryption schemes. X.400 tends to be deployed in networks with paid administrators, dedicated systems, managed user accounts, firewalls. Management model is highly collaborative. In terms of user notifications, X.400 has mechanisms that provide receive and read receipts.