Office365 sent multiple copies of messages.

Hi,

Earlier today we sent a mail merge using excel and Outlook 2007 connected to Office365.

However a short while later our customers started calling saying they were receiving 100's of copies of the same email.

I checked outlook and only 1 copy of the message is listed in the SENT items.  However, the Office365 delivery logs show hundreds of messages to the same recipients.

Why would this happen?

Thanks,

Abe