The error you will receive is:
Hi. This is the qmail-send program at
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

Sorry, I couldn't find any host named'. (#5.1.2)
This doesn't highlight the problem and your mailserver DNS will appear to be working fine when you check it.
The solution is to clean up your Outlook address book.
Step 1: Export the address book as a CSV file using these instructions
Step 2: Open it up in Excel, remove spaces and quotes from email addresses
Step 3: Save out as CSV overwriting the original exported file
Step 4: Import the address book back into Outlook 2010 using these instructions