Emails missing

I backed up my iphone 8 on my pc yesterday prior to getting a new replacement iphone 11. I had to give the previous phone back as it was on a lease plan. I restored the backup onto the new iphone and everything went smoothly however I have just noticed at all emails that were in my inbox on the previous phone are missing and only now showing emails received from the 14th August plus all my sent emails (sent from the previous phone) are missing and its empty. Why did all my email not restore and go missing, is there anyway to get them back?