Hi Melani,
That depends on what redirect you used. I would suggest you should use custom parameter with the redirecting url (Marketo URL)
Mapping: go.mycompany.com/directmail > go.mycompany.com/directmail?param=directmail.
You can then use smart list with "Visited webpage contains param=" filter to get the leads who came from directmail. Please test it once after imlementing.
If you have any other analytics implemented on your Marketo page e.g. Google Analytics, omniture, WT etc. you can get the statistics for the query parameter from there.
Please note:
If you used 301 server redirect then all the parameters associated with go.mycompany.com/directmail will not be captured and passed to Marketo page automatically. You need to do this mapping at server level.
Thanks,
Gunjan