We are using exchange server online with Outlook 2010 as the client.
Each time we try to send email to somone with an email address that match our domain we get an the error:
"This eamil cannot be delivered to firstname.lastname@example.org because the email address is no longer valid"
Over the last few days, Outlook users are suddenly getting the 'green' email address and the above warning when attempting to REPLY to SOME of our internal staff (different people are problems for different users)
I've tried the common fixes
-Delete the auto-complete individual address
-Delete the whole auto-complete cache
-Remove person from contacts and re-add
-Use online instead of cached mode (a few seconds delay before email shows as no longer valid)
-Outlook Safe mode (a few seconds delay before email shows as no longer valid)
Email DOES always get delivered though
As best I can tell, the senders email is the correct one
The issue is not seen using Outlook Web App