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How old were the emails that did not come across? Were they deleted or gone on your Post office inbox? Did you check your email clients if they were able to download it?
When you are on POP account, the emails that are stored on our server will be deleted once it's aged about 45 days. However, if the emails was downloaded by your email clients, you may be able to see it there, but it won't be visible in the Post Office.
Please provide us more information to analyze this case.
There were emails that were held in my In Box - and some of them were quite old, but still relevant.
It appears I incorrectly assumed that with the change from POP to IMAP my email would stay exactly the same. e.g. the content in all folders would all come across intact.
(No, they are not in the Post Office).