Try these steps.
First, reset the auto-cache following this method even if you already have:
1.Close Outlook, go to Start > Run > type "outlook.exe /cleanautocompletecache" and hit Enter. On Outlook startup, choose your Exchange profile. It will be loaded without cached entries.
Source
Then, follow these steps to fix the emails that don't work:
Source



The culprit I believe is the "mailto:" tags that are part of your contact's addresses. Clearing the autocomplete and making sure they are set to smtp instead of mailto should resolve this for any contacts you have or things you type not immediately failing. If this doesn't work we may have to come at it from another angle.

Thanks for all the excellent advise. I completed all of the steps. Still having a problem with my rberry@cadcourse.com. Even though I've created a contact for this e-mail address and use it in the e-mail, still the same result (below), sorry. None of my e-mail addresses use the mailto: prefix, just the straight emailaddress@domain.com.

Boy, this is so odd! Thanks for your help! All my best,
John

Your message did not reach some or all of the intended recipients.

Subject: TurboCAD v19 Primer?
Sent: 3/15/2012 8:21 PM

The following recipient(s) cannot be reached:

Rob Berry (rberry@cadcourse.com) on 3/15/2012 8:21 PM
None of your e-mail accounts could send to this recipient.