Hello Lawrence,
Thanks for the information on the this issue. Generally you would have an additional Content filter configured to detect the weighed emails. Can you please provide more details whether this is observed for all users on a domain or just a specific account. Do you have any other Spam or Anti-Virus application that may be executed outside of SmarterMail ?
Please send us a copy of the Delivery logs or SMTP of the next occurrence and we can review. Please open a Support Ticket if you would like for us to review the configurations on the account.
Thanks.