Outlook 2019 asking for password for non existing email

Hi All,

Starting the last few weeks, all of sudden I have this Outlook password pop-up problem.

I have been searching for solution without success.

Setup:

Windows Server 2012R2

Exchange 2013 latest CU 23 on premise.

valid SAN SSL for both mail.domain.com and autodiscover.domain.com

Outlook 2019 latest update.

Problem:

User ABC has email mailto:abc@domain.com

A few seconds after Outlook opened & connected to Exchange server, the password pop-up will shows.

Strangely the message shows: "Connecting to email mailto:xyz@domain.com "  (instead of mailto:abc@domain.com)

and it shows username: mailto:abc@domain.com and the empty password textbox.

The email mailto:xyz@domain.com does not exist in exchange (I've checked using get-mailbox command & search on EAC)

At this point the Outlook still shows Connected to Exchange server at the bottom right.

Even after click Cancel the Outlook still connected. And user can send/receive emails no problem.

I've tried to fix it by doing the following:

1. Remove password from Credential Manager

2. Create new Outlook profile.

3. Create new Computer profile & new Outlook profile.

4. Open Outlook in Safe mode

5. Disable antivirus.

Also notice this problem does not happen on the local network where the exchange server sits in.

It happens to a few remote users.

Any advise how to fix this will be greatly appreciated.

TIA.