chmod g+xs /var/spool/sms/outgoing/
If it do not help, then mention what are permissions and group of output file. If group has all rights i.e. r, w, and x, then you can add user smsd to group of /var/spool/sms/outgoing/ after setting SGID on /var/spool/sms/outgoing/. So any newly added file to /var/spool/sms/outgoing/ will get same group as that of /var/spool/sms/outgoing/ has and smsd will be able to perform any actions on that output file.