Invalid parameter when sending email to multiple emails

I have a user that at certain times when she send to multiple users, she gets "Could not complete the operation. One or more parameter values are not valid." error message balloon and she cannot send the email. I have already checked every single email address in the To: and found all the email addresses are complete and valid.

This error only happens occasionally only. Also attached screenshot of error message.
Oulook---parameter-v1.jpg
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shydocAsked:
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Davis McCarnOwnerCommented:
Almost every hit points to a corrupted autocomplete (NK2) file.  You can either simply delete it and let it rebuild or try to repair/recover it:

http://www.slipstick.com/problems/could-not-complete-operation/
http://www.nirsoft.net/utils/recover_corrupted_nk2_files.html
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shydocAuthor Commented:
Thanks Davis
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