Outlook for Mac 2016 - Repeated password prompts still an issue ...

I was experiencing the exact same problem, the solution is below, no more pain....

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

I was experiencing the exact same problem, the solution is below, no more pain....

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

It is now Dec 27th 2016 and we are experiencing this issue, after deleting the Keychain for Exchange it will prompt for normal login which it accepts but after a couple seconds we get the ADFS login prompt like loging in from home to Office 365 on Chrome or FireFox which we have never seen before for any Outlook whether MAC or Windows in native Outlook, usually once we login with the normal login it works fine unless account is locked or expire which still never shows a ADFS login - Very odd!!

Same issue for me - Outlook f2016 for Mac connecting to my O365 E3 Business account constantly prompting for credentials. I was an early adopter of Office 2016 for Mac and never experienced an issue until mid-Dec 2016. I cannot think of anything on the Mac that changed - no password changed and no updates to OSx. One evening both my iMac and MacBook started prompting for credentials within an hour of one another. If I re-entered my O365 password approx 8x (being prompted each time) it would finally take and work for about an hour and then start prompting again. Once it started prompting, I would not be able to reconnect it for about 30 minutes - after 30 minutes - rinse-n-repeat.

As a troubleshooting technique, I tried changing my O365 password, but that did not work. OWA did not experience the issue nor did my iPhone running Outlook for iOS.

I am running Sierra 10.12.2 and Outlook 2016 for Mac v15.31 on both machines (it may have started on a previous subversion of OL in mid-Dec but OL updated and the problem persisted).

I finally got around to contacting O365 support and they focused on whether or not I had multi-factor auth enabled (I did not), but they looked hard at it. They also focused on my Azure account. In the end, the fix was my Keychain. They said, they performed a repair/analysis on my mailbox, but found no inconsistencies. 

The following finally corrected my issue:

1)Closed Outlook on both my Macs

2)Opened Keychain Access on Mac #1

3)Selected 'Login' (top left pane) and selected 'All items' under Category (bottom left pane)

4)Deleted EVERY entry that referenced Microsoft, Office 365 and Exchange (I left the OneDrive entries)

5)Closed Keychain Access

6)Rebooted Mac #1

7)Opened Keychain Access on Mac #2 and ensured that the deletions replicated to Mac #2 and were removed. If any exist, deleted them.

8)Close Keychain Access on Mac #2

(Remember, Keychain access can be set to replicate between Mac devices. Yes, I know I could have probably deleted them from iCloud, but I wanted to ensure they were off the device)

9)After the reboot, logged to Mac #1, opened KeyChain Access and ensured that no MS/O365 related entries replicated back

10)Opened Outlook on Mac #1 and entered credentials.

This worked for me! I then opened Outlook on Mac #2, and entered my credentials and all was good on both Macs

PS: O365 Support had me delete my Outlook profile on Mac #1 before starting step #1 above, but I do not believe it was necessary because I did NOT delete my profile on Mac #2 and it worked fine.

Good luck to all because this was very frustrating.

Solution that worked for me.

OSX El Capitan 10.11.6

Office 365 + Outlook version - 15.29 (161209)

First off - this step might be unnecessary - but it worked for me.

Delete your Outlook account from Outlook 

(Not going to mention backing things up etc - that should obviously done before hand!!)

Next :

Make sure Outlook is closed - properly closed - use the top menu, or otherwise kill Outlook completely.

Open up Mac Keychain tool.

Choose "Login"

Remove *ALL* (delete) references to:

"Exchange"

"Microsoft Office Identities Cache 2"

"@<something>" that has a type of "MicrosoftOffice15**"

"Microsoft Office Identities Settings 2"

"MSOpenTEch.ADAL.<something>"

There should also be one listed as "unknown" - which if you double click on it - and look at the attributes - Microsoft Outlook is listed under "Always allow access by these applications"

I then re-opened Outlook, and as I had deleted my account - re-added account  - and pop - worked first time, and no re-pop-up of authentication.

Phew.... 

Microsoft and Apple shouldn't make this so hard.