/^localhost$/ 550 you are not me

# mx
/^{{ mx | regex_replace('^([^.]+)\..+', '\\1') }}$/ 550 you are not me

# nethence.com
/^{{ mx | regex_replace('^[^.]+\.(.+)', '\\1') | regex_replace('\.', '\.') }}$/ 550 you are not me

# mx.nethence.com
/^{{ mx | regex_replace('\.', '\.') }}$/ 550 you are not me