Outlook 2016 for Mac cannot connect to Exchange/Office 365

When logging into Exchange or Office 365 using Outlook 2016 for Mac. You receive the error “Authentication failed. Check your account information and try again” or the client simply stops syncing.

I looked around various forums without any response but found that the cause for me  was due to invalid KeyChain entries. You need to fix the KeyChain on your Mac by trying one of the following:

Test 1: Reset your iCloud Keychain password

If you are syncing Keychain Access to iCloud then try resetting the iCloud key by

  1. Opening iCloud under the System Preferences pane
  2. Click Options under Keychain
  3. Click Change Security Code and follow the instructions

Test 2: Reset default keychain

If this should not work then you may need to reset your Keychain

  1. Start the app Keychain Access
  2. Open Preferences and select Reset My Default Keychain