In some situations emails will be deliver successfully to one recipient but not to another, this is usually because they are being marked as spam according to the configuration of the inbound mail server. There are many different rules that mark an inbound email as spam, the most common is listed below.
The SMTP HELO command (when first connecting) is stating the hostname of the webserver (e.g. InfinitiServer01) instead of a fully qualified domain name such as intelledox.com.au. This can be changed by following the 'Set a Fully Qualified Domain Name' section in the following Microsoft article.