I have a hybrid configuration with my on premises Exchange 2007 box and Office 365 (almost all mailboxes migrated to 365).
I have recently added an Exchange 2013 server to my organization with another namespace. This new Exchange box is running in coexistence with the Exchange 2007.
This Exchange 2013 implementation was done because eventually we will be migrating our MX to office 365 and will decommission our Exchange 2007 box after doing so. The Exchange 2013 will remain in our organization hosting a couple of mailboxes (with a different namespace as mentioned) for some LOB applications.
To test email flow I have created a user and mailbox. This mailbox was auto created with a couple of SMTP address (due to exchange policies), one with the old namespace (the one hosted by my exchange 2007 and office 365) and another one with the new Exchange 2013 namespace.
I started to perform mail flow tests by sending and receiving. Everything perfect. Also opened the header of the emails received and verified that the box routing the emails was my new Exchange 2013 server.
Now in order to reassure that everything was getting routed using the new namespace, I deleted the old SMTP address of the user and left the one with the new namespace. I started to send test emails from the new user, worked perfect. Now, when I try to reply to that test email, I am getting an undeliverable message. this message reflects the test user email with the old namespace (the one I deleted) and is reaching my old server, which responds, "Hey I dont know this user". Even if I type it in the email with the new namespace, it will send to the old namespace.
The interesting part is if I send an email from an external email address it delivers to this new user with no problems, which makes me feel that my outlook client is suspicious.
Please enlightened me if possible!