hmailserver: message undeliverable

https://www.hmailserver.com/forum/viewtopic.php?t=22972
https://www.hmailserver.com/forum/viewtopic.php?t=246
https://www.hmailserver.com/forum/viewtopic.php?t=27158
https://www.hmailserver.com/forum/viewtopic.php?t=716
https://www.hmailserver.com/forum/viewtopic.php?t=24989

https://www.hmailserver.com/documentation/v5.3/?page=userguide_dds