Time of download shown in receive column rather than time of email receipt on server

I have an overnight email sent from one of my programs which, when sent via gmail smtp, showed the correct time/date received. Spamhaus didn't like this and I was flagged. Now, I have switched the smtp to my own domain, it shows the time I opened the email in thunderbird rather that the time the server received it. Looking at my email account via the web access, the received time is correct. By way of a test, I have Postbox on my pc and I added my account to that. It did the same thing, showing the time that Postbox viewed it, rather than server receipt time. I have searched on the web and, although I can find time zone issues, I find nothing like this. Can anyone suggest any areas of investigation I should be looking at? Many thanks.