Hey @Mart this is technically easy to set up but becomes difficult if there would be more than 1 deal open at the same time for a contact.
If there will only be one deal open for a contact at any time, you can use a CTA at the botton on your email. Then create a contact workflow to trigger on the CTA click, then you can set an action to “edit record” and set the deal as closed when the CTA is clicked from here.
If you’re expecting to have more than one open deal per contact this becomes a lot trickier. It’s hard to say an easy solution for this, but you might want to look at automating association labels on each deal to help segment them somehow, but you’ll need some way to segment a deal based on an association label within the contact workflow.