Error 552 5.2.0

Good evening,

today morning when I forwarded an email by outlook 2007, NIS 2012 showed ame a windows saying "552 5.2.0 string of char spam detected"

 

What's happened? My email is in spam? I was not able to forward my colleague the mail (only that particular mail; I tried to send an email called "ciao" and Norton did not give me any problem. Is the problem in the mail I wanted to forward? Which problem? Thanks

Francesco