Office365 - IMCEAEX Error

Issue:

User has a forwarding rule in Cardmail to send all messages to his closed Exchange account.

Use the below link for removing cached addresses from OWA, these instructions are the same for both OWA 2010 and OWA 2013
 
https://support.office.com/en-nz/article/Remove-suggested-name-or-email-address-from-the-Auto-Complete-List-in-Outlook-Web-App-9e1419d9-e88f-445b-b07f-f558b8a37c58

Because you had 2 accounts, some people, the ones that are getting the message (including you) need to clear out their address book, in order to remove the outdated entry for the exchange account. Even though it looks like they're using the correct address, they are still pointing to the wrong one.
To fix this you need to…
(Print these directions out)

 Go to on Outlook
File->Account Settings->Account Settings->Data Files-> Open File Location".
Close Outlook
Then delete the folder "Offline Address Books"
Go to Start
Type in Run in the search box if you do not see it
Type outlook.exe /cleanautocompletecache
(please note the space between exe and the /)
This will bring up Outlook again.

 
Try resending an email again and it should go through without issues.