https://www.msoutlook.info/question/reset-mailbox-sync-cache-exchange-or-imap

The search feature in Outlook relies on indexing the user's PST (personal store/database) files. If search indexing is disabled, search will be very slow.

In Outlook 2016:

1. Go to File → Options → Search (left pane)
2. Click the Indexing Options button
3. Click the Advanced button
   • May require admin privileges
   • Microsoft Outlook should be in the list
4. Click the Rebuild button
   • Wait until the process completes to test
   • The time it takes to complete varies depending on the number of emails and PST file size

Newer Outlook 2016 versions may not present the EAS option during account setup. A registry edit is necessary to disable the Simplified Account Creation and enable the EAS account type option.

https://wiki.zextras.com/wiki/ZxMobile_Outlook_Configuration

https://wiki.zimbra.com/wiki/Unable_to_create_Zimbra_profile_with_Outlook_2016

Exit Outlook before editing the Registry!

• Start a Command Prompt as Administrator

REG ADD HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Setup /v DisableOffice365SimplifiedAccountCreation /t REG_DWORD /d 1 /f

Or

1. Using regedit.exe, open this registry subkey:

   HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\setup

2. Create a new DWORD (32-bit) Value named DisableOffice365SimplifiedAccountCreation
3. Set the value to 1

To fix a confused Outlook:

1. Reboot the PC
2. Log in as Admin
3. Delete all saved credentials pertaining to Outlook or Exchange:
   1. Control Panel → Credential Manager → Windows Credentials
   2. control.exe /name Microsoft.CredentialManager
4. Log in as the user
   1. Don't open Outlook
5. Delete the user's Outlook Profile (or create another)
6. Recreate the Outlook Profile
   1. Do NOT ever select to save any passwords
7. Be careful to change the username on the second account when prompted with the wrong one!

http://social.technet.microsoft.com/Forums/en-US/outlook/thread/0b9942d3-78df-4421-9ebd-3d7df940f076/

http://support.microsoft.com/default.aspx?scid=kb;EN-US;970920

Outlook Safe Mode disables all plugins, which are the most common causes of problems.

In Outlook 2010 on Windows 7, you just hold down the control key when clicking the Outlook icon to start Outlook in Safe Mode.

In older (any) versions of Windows, you can run this command in the Run dialog box:

outlook.exe /safe

To determine which Outlook plugin is causing a problem:

1. Disable all plugins
2. Re-enable plugins one at a time
   • You probably want to enable the anti-virus plugin first

Disable all plugins:

1. File → Options → Add-in → Go (button next to “Manage: Com-in Add”
2. If there are any add-ins, clear the checkbox to disable them
3. Close Outlook, wait a few seconds, then restart it

Enable plugins one at a time while testing for problems:

• File → Options → Add-in → Go (button next to “Manage: Com-in Add”
• Enable one plugin (check the box) at a time
• Restart Outlook between tests
• Repeat the above procedure for each plugin you want to re-enable
• Once the problem reappears, you have found which add-in is the cause
• Leave the problem add-in disabled and either live without it or pursue a fix