Hey Michael,
In addition to wat Josh noted, you may need to flush your local DNS at the router level. You can see if the CNAME entry is currently available by opening up a local commandline and typing the following:
nslookup
set type=txt
[domain you're trying to reach]
This will query your local DNS to see if the entry is available. If it's there and is pointing to your Marketo instances account URL, then it s not explicitly a DNS issue, and may be related to other netowkr restrictions in place.