Mail addresses with more @ signs

[Lasantha Marian]

Friends,
I have noticed that MailScanner ignores messages with envelop address 
having more than one "@" sign. Notably those messages are appended with 
an IP address. An example like (sender at xyz.uv@193.216.123.213) returns 
"X-ABC-MailScanner-SpamCheck: spam(no watermark or sender address)".
Any thoughts as to what could cause this (letting this spam go as ham). 
Looking at the message content, they are positively spam.
Best regards.
--
Lasantha.