"WARNING - NOT VIRUS SCANNED" issue

Hello,

Recently, a user complaint that an email message shows "WARNING - NOT VIRUS SCANNED" in the subject line while the same senders did send to him many time in the past without this problem. I can figure out the sender should be using Lotus Notes 7.0.3 version. 

[quote]X-Mailer: Lotus Notes Release 7.0.3 September 26, 2007[/quote]

I know that the malformed MIME message will be labelled as "unscannable" message. However, Lotus Notes should be one of the famous email client which supposely not created the malformed MIME message. Do you think there is any other possibilites causing the problem? 

PS. We did not change any configure in the past year. 

Thanks! 

Jeff