zimbra "calculates" wrong url

[stegbth]

hi,

our zimbra allows only access with https.

so it got set to
zmtlsctl https

as the server get connected from an external with an different url i added
under domain "hostname of the public service" the other hostname.

when i now open a mailbox out from the admin console, the hostname is correct as needed, but the protocol is set to http, instead of https.

so i thought to prepent the protocol before the hostname, then i get
http://https://myhostname.domain.com

which doesn't work also

where is my error?

greetings
thomas