A bit of background on our Exchange setup. We use Office365. We have 10 accounts in the E1 Plan, and 2 domains (Let's say a.com and a.org). a.com is being hosted on premises, a.org uses the E1 Plan. Both domains' DNS are via Hostgator. Our plan is to move from a.com, shutdown the hosted exchange, and move entirely over to a.org email. We will continue using a.com for a few months until everyone is aware of the change.
I have created mailboxs for the a.org account, and added a.com as an addon domain via Office365 Admin. I have created the a.com email as an additional email to the corresponding mailbox in a.org so emails sent to a.com or a.org go to the same mailbox.
On the user side, they are using Outlook 2010. Each user has the a.com as one mailbox, and a.org as another. The a.com mailbox in Outlook is from the time when only a.com was used, the plan is to copy the .pst over to a.org and migrate old email, calendar etc as its only a few accounts.
Everything tested working fine via Outlook Web App and Mobile devices except for Outlook. For Outlook 2010, receiving email is fine from all domains, but when the user replies to emails from a.com or a.org, it sometimes fails. Emails to a.org sometims 'sits' in the Outbox even with several 'Send/Receive' attempts, emails to a.com bounce back with the message 'This e-mail message cannot be delivered to Name Removed X because the e-mail address is no longer valid.'
I have a feeling it's something with Outlook and what's being configure there - the same user can send/receive emails fine from the same computer while on Outlook Web App and on their phones.
Help appreciated.