... that was the answer when I tried to send a message to firstname.lastname@example.org - I was really wondering ... I work with Ubuntu, use a antivirus ... well ... it meant I had to scan my computer, so I did - no result, everything fine.
Then I tried to send the message with the yahoo.it web interface: same result, I was told I had a virus. That sounded really weird ...
Then I looked at my message, the subject was "Photos" because I wanted to thank people for sending me photos. And I thought ... let me send a test message. So I sent a message to the group using "Test" as subject. That one went through without any complaints.
Therefore the yahoo server considered my first e-mail(s) to have a virus because in the subject line I only used the word "Photo". Weird, right? ... So don't do what I did :-D