...the following:
if Sum ({MSG_DST_ENTERPRISE__PROTOCO_VW.Total_msg}, {MSG_DST_ENTERPRISE__PROTOCO_VW.DATE}, "daily") <> 0
then
Sum ({@SMTP}, {MSG_DST_ENTERPRISE__PROTOCO_VW.DATE}, "daily") *100/
Sum ({MSG_DST_ENTERPRISE__PROTOCO_VW.Total_msg}, {MSG_DST_ENTERPRISE__PROTOCO_VW.DATE}, "daily")
else
0