“I have got version 4.8.8 by Mike Walsh & Marvin Labbs installed” 4.8.8 of WHAT?
As far as I can see, the plugin is simply called “Email Users” by Mike Walsh & MarvinLabbs. Does this help? If not, any idea where I can find further info on it? When I click on the list of Plugins on my Dashboard, and then on “view deatils” under the relevant plugin, that’s what it says.
I recommend asking at https://wordpress.org/support/plugin/email-users/#new-post so the plugin’s developers and support community can help you with this.
Note, however, that the plugin is over 4 years out of date. Probably time to find another.
I did as you advised, but there’s no reply so far in the relevant plugin’s forum. Now, this may sound like a very dumb question, but how do I go about replacing the plugin with another? Can I simply remove the old plugin, then go and find a new one and add it? Sorry, but I did not create this webpage, I took it over from someone else, and I am learning step by step from scratch.