I check accounts from two different Exchange domains with Outlook. Check might be a bit of an overstatement for one as it is maybe once a month and more like every couple. Both require me to periodically change the passwords. And both send daily email warnings leading up to the deadline.
Because I might miss the week of daily warnings for one, I set up a rule in Outlook to forward the message to myself. Thankfully I set it up to send to more than one email account. I got the warnings in just one place which bothered me.
The missing messages had bounced. The address listed in the error looks nothing like the address in the rule. (I would post it, but I have no idea if that would compromise the account.) My guess is Outlook does not firewall the contacts in rules. So if I enter an address for the cross domain and it recognizes the address, then it shows me in the UI the human friendly but the backend uses the Exchange friendly one.
My solution ironically was to leverage a third party. My web hosting provider allows me to create unlimited email addresses, which I use to set forwarding accounts. I created one for this purpose and replaced the problem one in the rule. It works now.
Also: CHANGE YOUR PASSWORDS!!