I noticed that you had not gotten a response yet on this so I thought I would add something.
With the new features of UE4 you can send email 'blasts' that will send so many emails within a timed interval. This feature makes it so you can send out things like newsletters from your live server without overusing the resources available.
There are a few ways you could do this but the simplest would be to include a link in the footer of the message that links back to your site with an identifier for the user, like their id. You would then automatically update the record to indicate that the user is no longer subscribing or remove them from the db. Doing it like this would require using a tool like DataAssist to craft the update or delete page that the link in the footer will go to.