rcpt_to.access.whitelist_regex

# Does the same thing as the whitelist file, but each line is a regex.
# Each line is also anchored for you, meaning '^' + regex + '$' is added for
# you.  If you need to get around this restriction, you may use a '.*' at
# either the start or the end of your regex.  This should help prevent people
# from writing overly permissive rules on accident.