Here's what the user said when they send a large attachment:
An unknown error was returned from the SMTP server. Subject 'Test',
Account: 'moment.net', Server: 'mail.moment.net', Protocol: SMTP, Server
Response: '432 Service temporarily unavailable.', Port: 25, Secure(SSL):
No, Server Error: 432, Error Number: 0x800CCC61
If I use a feature in Outlook Express which splits the file into six
parts, then I am able to send it.
Check your syslog. There should be some additional errors there.