How Do I Fix Outdated Email Addresses in Outlook?

What you’re dealing with has to do with the legacyExchangeDN used for recipient resolution within Exchange. When a recipient is deleted and then recreated, it can cause these outdated addresses to pop up. The best way to mitigate this is to take the legacyExchangeDN value for any recipient that’s getting deleted and apply it as a proxy address to the new recipient. If you're stuck, you can convert that IMCEAEX string into a legacyExchangeDN string and then add it to the new recipient's addresses. This should help fix the problem.

I had a similar nightmare with IMCEAEX entries about six months back. They really stick around like a bad habit! While NK2edit is a decent tool, it sounds like you might need to look deeper into the Exchange settings. Usually, these entries come back if there are lingering legacyExchangeDN attributes or mail-enabled objects that weren't cleaned up during a migration. I’d suggest running 'Get-Recipient' on the emails causing trouble to see if there are any phantom objects. Switching your Outlook to online mode might also help you figure out if the issue is client-side or server-side caching. Good luck!

Did you experience this after migrating mail from another mailbox? I’ve run into similar issues where Outlook caches contacts with their previous X500 address. If any recipients were deleted and recreated during the migration, they may have new addresses, while the cached versions are outdated. A good approach is to take note of the original X500 addresses and apply them to the new recipients. You might also need to delete and re-add contacts when replying to emails to prevent the issue from continuing.