Try this - assuming you have the Style Property of the Listbox set to "Checkbox"

Code:
Dim intDummy As Integer
Dim strSendTo As String
Dim strShellCmd as String

strSendTo = ""
For intDummy = 0 To List1.ListCount - 1
    If List1.Selected(intDummy) Then
        strSendTo = strSendTo + List1.List(intDummy) + ";"
    End If
Next

If strSendTo = "" Then
    MsgBox "You didn't pick anyone!"
    Exit Sub
End If

strSendTo = Left(strSendTo, Len(strSendTo) - 1)

strShellCmd = "start mailto:" + strSendTo
Shell strShellCmd