After clicking on a link within a sent email, link gets a 404 redirect error instead of going to the expected landing page. However when clicking to that link within the Marketo Email Editor it goes to the expected landing page without error.
This happens when the tracking link domain name is pointing to the wrong Canonical Name (CNAME). The reason it works in the previewer is that the previewer does not use tracking links.
Run a CNAME lookup on a tool like mxtoolbox.com.
Take note of the Canonical Name
Double check that it is the same as the "Tracking Link" found under Admin/Email/Tracking Link.
If the CNAMES are not the same you will need to work with your IT team to modify them.
Once this has been modified and propagates through the web (24 hours or so) tracking links should work.