I'm searching for a solution to sync contacts and calendars in standard clients for desktop and for mobile. It's really unbelievable that nowadays an open source project remains monolithic without allowing a connection from outside. With opener 6 I was able to sync with caldav and carddav procedure. I was also able to use an external imap server different for each user. Actual interface is wonderful, but the software is missing a lot of features believing to be the only place where users must connect. I can be wrong, so I'd like to learn how to get those connections working, since they are necessary to use properly any kind of erp.
3 years later and there's a solution to sync contacts (but not calendars or emails at this time) 2-way between Odoo and other apps, like Google/Office365 contacts, Mailchimp, Hubspot and many other apps. Here's a list of apps we currently support: www.piesync.com/odoo/
Hope this helps!