I used to send mail merge emails with personalised attachments using Outlook 2011 for Mac. Now I can't.

The crazy thing is I get two different error messages depending on which account is the default in Outlook. This for a Google account:

Delivery to the following recipient failed permanently:

    *** Email address is removed for privacy *** 

Technical details of permanent failure: 
Internal parse error: Illegal envelope To: address (invalid domain name):   *** Email address is removed for privacy *** 
and this for an IMAP account:

I've uninstalled and reinstalled Office 2011. I've set Mail as the default client. Quit, restarted and set Outlook as the default. I've searched the web for similar problems but cxan'tr find an answer.