Our client uses Microsoft Office 365 for their hosted email. The one user on a Mac is having issues today sending to multiple recipients in a group list. They are using Outlook for Mac, not Entourage. The error message is "one or more recipients are invalid" error code: -19841. We have already created a new group with the same issue. All the recipients are in the local domain. We can send to individual users without issue. A web search showed little information on this error.