Followup (belated), mailserver running NethServer in VM has been chugging away for weeks with no complaint! I was able to copy maildir contents from Zentyal virtual drive to NethServer virtual drive, after getting help with the pathname to use. All the old emails were then accessible, either via mail client or webmail.
I’m grateful to the community for their quick responses to requests. Next time a mail server is needed, I’ll start with NethServer!
Consider this thread resolved. I’ll be happy to help further to document my transition from Zentyal.