Is it possible to provide webmail service for domains that exist in Plesk but have third-party mail configured so that mail is forwarded to external mail server?
This functionality is not supported in Plesk at the moment. The new functionality can be suggested in Plesk on Plesk User Voice portal:
It's possible, however, to configure Horde webmail to authenticate on external mail server, but this solution is not supported in Plesk:
Note: Webmail can work only if mail service is turned ON for a domain in Domains > example.com > Mail Settings.