Hi Bob,
you may want some letters of Basic Latin U+0041 - U+007A
Just give it a try, but it should work:
set email "steve-o@jackass.something.com"
regsub -all {([u0041-u007A])} $email {&#[scan %c];} result
puts [subst $result]
Results in:
steve-o@jackass.something.com
(
Look at the HTML source of this message to see it 😊 )
Hanjo.