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I am using Windows 8 with Outlook 2013. I have one Exchange (Office 365) account and one Google Apps account using the Google Apps Sync software.

Occasionally, when replying to an e-mail or writing a new e-mail, I'll get an error reading "Messaging interface returned an unknown error. Cannot resolve recipient." It doesn't matter which account I try to send it from. However, the common thread does seem to be that it only happens with recipients not in my address book. Adding a recipient to my address book does seem to resolve the issue.

Searching online, I found suggestions that this could be caused by a corrupt PST file and used a tool to correct my PST file. This made no difference.

How can I resolve this? Adding recipients to the address book or logging in through the Gmail Web interface are workarounds but not really satisfying solutions.

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I had the same problem and i think i figured it out... I removed all of the BUSINESS FAX Numbers from the contact cards and the emails began sending as normal! I think it is an issue when sending emails to a group of people that have fax numbers stored...

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There are a few reasons I don't think this is my problem: 1. I don't have any contacts with a business fax. 2. This happens whether I am sending to one person or many. 3. The determining factor is whether each contact is in my address book at all. –  emodendroket Dec 18 '13 at 20:01
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