No. All the email is scanned and assigned a score based on the likelihood that an email is spam. Thresholds (low scoring and high scoring spam) are used to determine whether an email should be tagged as spam: it is important that this is done to help avoid false-positives and false-negatives.
No system can guarantee 100% detection, though nearly all infected files and dangerous file attachments should be detected using this service. The service scans all email received and sent through the server to help ensure that you do not accidentally start spreading a virus yourself.
On most domains, default settings are:
Deleting high scoring spam -- tagged {Definitely Spam?}
Allowing low scoring spam -- tagged {Spam?}
