Email is not built into the OO suite. However the closest answer to a default email client, in my view, for OO would be Thunderbird (
Thunderbird - Reclaim your inbox) and to add full scheduling capability its email plug in Lightening (
https://addons.mozilla.org/en-US/thunderbird/addon/2313).
Both are from the Mozilla foundation- creators of Firefox.
I have used OO and Thunderbird for years, adding Lightning when I went freelance a year ago. Neither I or more importantly my clients, have missed MS Office.