SMTP vs. POP3 Scanning

[Matt]

When a new virus comes out it could be hours before a signature is available
for the virus scanner and in that time it could slip into many mailboxes.
When it does get in the virus scanner database it does nothing for the mail
already in mailboxes.  Many users only check there email once a day if that.
Would it not be an added benefit to scan at the POP3 phase as well as SMTP?

Just a thought and sorry if this has been covered before.  I have got emails
from a user before that had his virus scanner catch viruses mine missed.  I
tell all users they should still keep there virus scanners up to date.  But
as well as ClamAV + MS has worked for us many don't.

Matt