$mime->header_set('X-Alt-Message-ID', '<alt-id-for-nntp>');
is_deeply(['mid-1@a'], mids($mime->header_obj), 'mids in common case');
$mime->header_set('Message-Id', '<mid-1@a>', '<mid-2@b>');
$mime->header_set('X-Alt-Message-ID', '<alt-id-for-nntp>');
is_deeply(['mid-1@a'], mids($mime->header_obj), 'mids in common case');
$mime->header_set('Message-Id', '<mid-1@a>', '<mid-2@b>');