@subdom..dom.ext

echo "@dom.ext" | grep "^@[[:alnum:]]+[[:alnum:]-.]+[[:alnum:]]+.[[:alpha:]]+$"

@subdom.dom.ext
@subsubdom.subdom.dom.ext
@subsub-dom.sub-dom.ext

^@(([a-zA-Z](-?[a-zA-Z0-9])*).)+[a-zA-Z]{2,}$

for d in @subdom..dom.ext @dom.ext @subdom.dom.ext @subsubdom.subdom.dom.ext @subsub-dom.sub-dom.ext @x.org; do
 [[ $d =~
    ^@(([a-zA-Z](-?[a-zA-Z0-9])*).)+[a-zA-Z]{2,}$
 ]] && echo YES || echo NO
done

^@(([[:alpha:]](-?[[:alnum:]])*).)+[[:alpha:]]{2,}$

grep '@[[:alpha:]][[:alnum:]-]*.[[:alpha:]][[:alnum:]-]*.[[:alpha:]][[:alnum:]-]*$'

echo "@dom.ext" | grep -E "^@[a-zA-Z0-9]+([-.]?[a-zA-Z0-9]+)*.[a-zA-Z]+$"