The import21 command doesn't make regexps for acceptable_aliases.
In Mailman 2.1, the entries in acceptable_aliases are always regexps matched with re.match. I.e. an entry like other_name@.*$
is interpreted as a regexp equivalent to ^other_name@.*$
. In Mailman 3 acceptable_aliases are either regexps or literal strings depending on whether they begin with ^
or not. Thus, import21 needs to prepend ^
to the values.