Having created a list of some 57 individuals from my LiveMail contact list, I crafted my message and pressed Send - I then get the error message:

This message could not be sent.
Some of the recipients for this message are not valid.
Please verify the names

Since this message tells me nothing about which of the 57 recipients is causing the problem (nor why), how can I resolve the problem other than by a lengthy trial and error exercise? 

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