Easiest way is probably a connection to a Salesforce campaign, plus Interesting Moments. Create/update the Direct Mail program tag to the statuses:
- Mailed
- Visited
- Converted
Where Visited means they visited the site or the specific web page, and converted means the lead went to sales (MQL) or was added to an Opportunity.
Let Marketo create the Salesforce campaign so these statuses will appear in SFDC and will be automatically updated.