The Nabble part is straight forward enough.
Any forum user can get copies of any post delivered direct to their mail program as it is made simply by selecting:
Options > Subscribe via email
What happens after that is down to the facilities available in your email program...
Good ones will allow you to set up filters so that you can refuse to accept certain mails, or to direct them to a folder, of your choice, within your InBox or send unwanted mail to your Spam box. It all depends on the number of posters you are interested how you set up the rules.
It's not quite the same as "tagging" but, depending on the frequency and volume of posts you receive, it can be every bit as effective, as Nabble also allows you to respond to those emails by email too, which could mean you never need to visit the forum itself again!
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.