As it turned out, if Lync and Outlook are started at the same time, changing Offline mode is not saved after Outlook is closed (Lync somehow blocks saving offline mode settings, maybe, due to its constant connection to Exchange).
TURN ON CACHED EXCHANGE MODE OUTLOOK 2013 SOFTWARE
Then select Software > Microsoft > Office > 12.0 > Outlook > OST. TIPS: The steps are similar in Outlook 2013 but, for Outlook version below 2010 like. Under the Server name, uncheck 'Use Cached Exchange Mode'.
In E-Mail settings choose the Server and opt 'Change'. Select Account Settingsfrom File option of Outlook. Alternatively, click on Start and then Run. Follow the steps to turn off the Cached Exchange Mode. A user has to go through the following set of instructions to enable the cached option: Press Windows key+R.
The solution was unexpected: all users who reported Outlook start offline problem had Skype for Business (Lync) installed. Solution To Enable the Use Cached Exchange Mode 2016 Option. We reinstalled and repaired Outlook – nothing helped. We tried to delete an Outlook profile and re-create it again.We started Outlook in the Safe Mode ( outlook.exe /safe command) and disabled all Outlook add-ons: the problem persisted.In Exchange 2013/2016, the main protocol for connecting Outlook clients to CAS is HTTPS ( MAPI over HTTP), so it suffices to check the availability of port TCP/443 You can do it using the Test-NetConnection cmdlet: tnc ber-msgcas –port 135 – the port is available. In Exchange 2010, you need to check the CAS server availability from the user computer thought port TCP/135 (RPC locator).The accessibility of a mailbox through the OWA interface: the mailbox was available.