Some of my clients are not receiving my emails

Some of my clients are not receiving my emails

I have sent lots of clients emails via Zoho CRM and lots of them are complaining that they are not receiving them so I end up sending it to them not using zoho, they seem to get these. 

I have spoken to Zoho Support about this issue loads of times and they say no problem exists and all my emails have been delivered.

I understand sometimes email end up in junk mail so may be missed, but this can not be the case for every single one

Is anybody else having this problem?