My experience is that it ist, like you mentioned, the link tracking that Marketo appends.
Happened to us too, links were fine one day, and the next, every link we sent via Marketo email to our own domain (internal employees) was blocked. Our IT team had to change the security settings within the entire organization.
We found out that with most organizations, what's blocked and not blocked changes regularly, and in your case, the DHS may have upped their securities prior to your sending the webinar follow-up. To temporarily get around this, you can add a class of "mktnotrack" to any of the links in emails that your organization sends to the DHS domain. You won't get the luxury of the click-thru information that Marketo provides, but at least the pages won't be blocked. See the tutorial here: https://community.marketo.com/MarketoArticle?id=kA050000000KyqCCAS
I'm not sure you have the luxury of asking the DHS to loosen securities for Marketo emails, so I'd open a support ticket, if you're able.