Hi ZoranMarjanovic,
What do you mean by “offline Outlook mailbox(Outlook caching)”? Is it Cached Exchange Mode in the Outlook client?
If yes, I would like to tell you since E1 plan is an Office 365 Exchange account, it is available for E1 plan account to use Cached Exchange Mode in the Outlook client.
For more information, please refer to:
support.office.com/.../Turn-on-Cached-Exchange-Mode-7885af08-9a60-4ec3-850a-e221c1ed0c1c
If I misunderstand your meaning, please post back your detailed requirement, we are always here to help you.
Best regards,
Mouran