Using classic Outlook for Windows in business environments
Ok, so working with support, here's what worked for me.
Delete all mailboxes from new outlook
Change Microsoft account alias from custom @xyz.com back to default @outlook.com
(I had to walk away and at least 12 hours passed - no idea if that was needed, just putting it out there in case you try right away and it fails)
Launch new outlook
Add Microsoft @outlook.com mailbox
Add @xyz.com mailbox (for me this was IMAP)
Change Microsoft account alias from default @outlook.com back to custom @xyz.com