Not receiving only some email ...not spam but customers

Not receiving only some email ...not spam but customers

Hi.
I've recently started to settle into using Zoho email in CRM and it all seems to be working fine except that i noticed recently that some emails i have been sent are not coming through.
I still have outlook operating on my PC so i can compare what is received in Zoho with those emails from outlook and they should all be the same.
Most are however as i mentioned already i have at least 2 cusotmers whose emails are not being received in Zoho but they are coming through in outlook.
Everything else is working fine and all other emails including the usual spam is making it through to the zoho account.
Both are for the same email address.