The error you will receive is:
Hi. This is the qmail-send program at mail.allipo.com.
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.

<'[email protected]'>:
Sorry, I couldn't find any host named somemailserver.com'. (#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
http://office.microsoft.com/en-us/outlook-help/export-contacts-HA101870639.aspx
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
http://www.addictivetips.com/windows-tips/how-to-import-contacts-from-excel-file-to-outlook-2010/
I found this solution here: http://forum.parallels.com/showthread.php?t=80725