In this situation, X.400 software had been working without problems. Suddenly, the message exchanges failed in ma_start_delivery(). It was returning an error code of 100 (BAD_MESSAGE).
The ma_start_delivery() call fails when trying to exchange a file of more than 900 bytes.
X.400 was restarted with the wrong umask. To fix, set the umask to 0022 and restart the software.