Welcome to the forum Tony!
By chance, have you downloaded any new apps recently? You may also try going into your applications manager and clearing the cache under the email client you're currently using.
That's about all i can suggest at the moment. Hopefully someone else will be able to offer some more suggestions.
Good luck! 
Thank you CR6 for your reply.
I have not installed any new apps lately.
I went into the Applications Manager but could not find any email client listed there. I use gmail and BigPond (Telstra Australia) for emails but could not find any cache associated with the settings for these clients to delete.
My provider (Telstra BigPond) has suggested I only use the phone for calls and SMS until my recent data usage can be assessed at the end of the "month" which will be on 20 Sept 2012. They have switched off data access and suggested that I do not use Wi-Fi until the
rogue app (my interpretaion) can be identified.
My feeling is that the problem was associated with Telstra rather than Wi-Fi as the Telstra 3G or 4G data allowance suddendly increased at the time of the overheating.
I look forward to any further ideas from you and the forum. Thanks again.