I got a reply from HTC in regard to the sync issue we've been having, see below
Thank you for contacting HTC regarding your HTC One X.
My name is Ryan and I am a Technical Support Agent for the HTC Written Team.
I’m sorry to hear that you are experiencing difficulties with Hotmail application saying that sync is unavailable when trying to sync you Hotmail account with the stock mail application.
The reason for this is that a Hotmail account sends so much information back and forth it makes the stock mail application struggle to refresh the non important information and cache.
To resolve this, John, I would recommend continuing using the official Hotmail application from Microsoft as the application has been specially designed to clear/refresh this mass amount of information and cache. The stock mail application on the phone has the properties to refresh the information that needs to be cleared but the Hotmail account sends so much information back and forth the stock mail application struggles to clear this information preventing the phone to sync with your Hotmail account.
The reason the Hotmail account works fine when you delete and re-add the account is due to the phone completely clearing the information from the account and then setting up a new memory bank for the account causing the account to sync fine until the information that the stock email application is unable to remove and over time this clogs up the application cause this sync failure.
We are looking for Microsoft to update the information management making it more manageable to handle on mobile email applications.
I'm sorry if this cause any inconvenience to you.