thanks to your replies (and gentle chiding) here, the mail can now be consulted remotely.
It was surprisingly easy, albeit with extra twist courtesy of our ISP
1. ask the domain hosting co to add a cname record to the dyndns ext. name
2. add router rule forwarding a external high port to the server 443 port
(1,2 because ISP will only sell static IP and open 443 port at exorbitant high prices)
3. restart zimbra in https only mode (zmtlsctl)
4. add exceptions for selfsigned cert in firefox
5. open
https://cname:highport
and presto, zimbra is here (and way over there

)
Just one follow-up question:
Can I disable the automatic sending of the certificate by the server?
I'd like install the certificate manually on the laptops.
Incoming requests to the server would not get even to the login screen if the certificate is not already present on the caller's machine.
best regards
glenn