CalDAV
Windows 10 supports CalDAV/CardDAV but limits provider-support
Windows 10 supports syncing email, contacts, and calendars with all major providers. However, Microsoft chooses to close it down to a handful of providers.
DNS
DNS SRV records usage for email and calendar service discovery
A quick survey of the use of DNS SRV records for auto-configuration of email and calendar hosting services. Provider support is fairly good.
How to install and configure Horde 5 webmail on Debian/Ubuntu
Step-by-step instructions for setting up the Horde webmail and application suite on Linux.