An "Undeliverable" email response from mail sent to a valid internal DMC employee is often the result of an outdated email address stored in your Outlook client auto-complete cache.
Removing this outdated address is simple and usually resolves the undeliverable issue. Please follow the steps below:
Open a New email message
Type the first few characters of the email address or person's name for which you received the "Undeliverable" message. Outlook will automatically "suggest" the email address in the dropdown.
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When the entry appears in the suggested names, move your mouse over the name until it becomes highlighted, but do not click the name.
When the "X" icon appears next to the highlighted name, click the X to remove the name from the list. See image above.
Now type the full email address of the intended user and after successfully emailing them, the auto-complete will be updated with the correct address.
Note: It's also helpful to re-download a fresh copy of the Global Address Book from the server. See; Outlook Global Address Book Download