Too often, my outgoing emails end up in the Outbox allegedly because "The message could not be sent because it has unresolved recipients." I verify each recipient but to no avail. Yet, when I send the email directly from Comcast, it sends just fine, with no "unresolved recipients" error message.

Is this a problem in my preferences or a glitch in Outlook? 

I use a MacBook Air with Outlook: Mac 2011.