Although Shield blocks and deletes at least 90% of unambiguous spam, it continues to deliver lower-scoring probable spam and suspected virus mail (after cleaning it, of course) because no automatic scanner can be perfect — a very small percentage (far less than 0.1%) of legitimate mail will be tagged, and users wouldn’t want it deleted. You can either filter the delivered [SPAM] and [VIRUS] mail visually and delete it by hand or set up automatic filters in your email software to automatically place these messages into a junk folder.
If you are a Shield user and are seeing spam slip through untagged, you likely need do nothing but wait a week or two for the filtering software and databases to catch up to the spammers. Likewise with viruses: the rare variety that slips through will almost certainly be reported and added to the scanner within a matter of hours. However, if you see even a single false positive (legitimate mail tagged as spam/virus), we want to know about it right away. Please see the FAQ for more detail, or speak to your domain’s email administrator.
If senders receive a rejection when sending to you that says they’ve been “greylisted”, this is an indication that their mail server is not fully compliant with Internet standards. For more detail see the FAQ.