MOST emails with ZOHO send mail do not reach the recipients

MOST emails with ZOHO send mail do not reach the recipients

This is worrying me a bit.
Confirmations of receipt after filling out a ZOHO form do mostly NOT reach the recipient. I've seen it happen on all platforms except on Macs (they all seem to receive fine) however 8 out of 10 receive nothing.
Can someone perhaps tell me if they have had this problem and what they would do about it.
twestc