How to replace some characters after the last dot with their lower cases

A simple question but I cannot solve it. I got a string like this mail address:

ma <- "something@somewhere.COM"

My goal is to get:

"something@somewhere.com"

So to put the to lower cases the part after the last dot. I've read this, this, so I tried:

gsub(".*\\.","\\L\\1", ma, perl = T) 
[1] "COM" # nope

Also something like:

library(gsubfn)
options(gsubfn.engine = "R")
gsubfn(".*\\.", ~ tolower(x), ma)
[1] "something@somewhere.COM" # nope

I'm quite confused because it seems I can fetch the part I want to replace:

gsub(".*\\.","", ma)
[1] "COM"

But I cannot replace it properly. If you can give an explaination with the solution, I'll gladly appreciate it, regex is not my strongest feature.