permit_mx_backup_networks uses the same parser as mynetworks, and therefore it expects the same main.cf syntax as mynetworks.
In smtpd_check.c: mynetworks = namadr_list_init(match_parent_style(VAR_MYNETWORKS), var_mynetworks); ... perm_mx_networks = namadr_list_init(match_parent_style(VAR_PERM_MX_NETWORKS), var_perm_mx_networks); > [2001:6f8:128a::1]/128 # dict_open: unsupported dictionary type: [2001: This does not reproduce here with: permit_mx_backup_networks = [2001:6f8:128a::1]/128 Wietse