To add a bit more information: 

I just discovered by sheer accident that I am able to email under the following condition:

1 open an invoice from the customer list in Customer Center; I click on Email. When the Send Invoice box opens, I click on send. I still get the message that states "Quickbooks can't complete the current action due to a missing component", but when I close the message box, I get the message "Message was successfully sent".

When I try to email from the File ==> Send Forms to send multiple emails, I get the message about the missing component and none of them are sent.

I don't know if this will shed any more light on what the issue is...

Please let me know your thoughts. 

Thank you.