After reading some source I see that (at least) the smpp implementation does not handle multiple receivers (seperarated with space). (this however works with cimd2). Therefor i suggest that the following is removed from the documentation:
Phone number of the receiver. To send to multiple receivers, separate each
entry with space (' ', '+' url-encoded) - but note that this can be
deactivated via sendsms-chars in the 'smsbox' group.
