Hi @Sfalzon ,
Welcome to the community!
For POP3 email accounts, it is by default that emails will be deleted after 45 days. The only way you can retain the emails is to use an email client and download all those email to your device.
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.
If your was sync to iCloud you may try to check it with iCloud server to retrieve. Unfortunately we do not have a way to retrieve the emails you sent using the device.
Let us know should you require further assistance.