That is an valid IPv6 address. You can block it in the IP Firewall and you can also use the protect my login rule (or a custom rule) in Rate Limiting to block such attacks more broadly.
I know a little about IPv6, but didn’t realize this address was one! Duh. I think what stumped me was the fact that nothing at all came up when I entered it in Google search. The older IP versions always came up with pages of information about location, abuse, etc.
Regardless, thanks so much for explaining this and telling me another more efficient way to block this sender. Will add it now and then explore Limiting for a more efficient solution (hadn’t known about it before since the rules feature kind of confused me…not too technical, I’m afraid).
I appreciate your quick response as well as your practical suggestions.