Can you tell us what type of attachment it is ? I can see that it detects a .exe but was it a .zip that has been extracted and run ?
e.g. is it a .zip, .exe, .doc, .bat ?
Also from the command line can you paste the output of :
db mailpatterns show
config show clamd
config show clamav
freshclam -v
or
clamdscan -V
That should show how your server antivirus is set up.
Remember that the best antivirus system is an educated user