Some emails not being delivered to our users

I am seeing an issue where some emails are not delivered to the users, at least I cannot find them.

For example today one of my users had their PC hacked and encrypted so I am attempting to reset their Amazon password for them and requested to chnage the password which requires the entry of a validating code sent via email.

I am in their email acount and the code did not arrive.

I checked our anti-spam gateway through which our email goes and is delivered on to your SMTP servers and I have a confirmed acceptance of the message by SOHO's email server. see below:

SMTP Delivery Result

Created on 13-Feb-2019 14:57:37

Status Success

Message mx.zoho.com:204.141.42.121 accepted message. STARTTLS proto=TLSv1.2; cipher=ECDHE-RSA-AES256-SHA. Remote host said: 250 Message received

However the message is not in the recipants mailbox.

This isn't the only example of this kind of problem we have seen, it seems there must be something wrong.

Regards

Gary