Email history lost when upgrading to 13.1

To begin with, POP3 was never designed to be used with multiple clients.

All mail downloaded from a POP3 server resides locally on the device.

You may be able to restore the device from the backup made prior to updating.

Going forward, I have two recommendations:

1. Switch to IMAP or Exchange instead of POP3. POP3 is largely considered obsolete. Both IMAP and Exchange will synchronize all clients with the server, ensuring that changes made in one client are reflected in all others. Besides which, if your phone gets run over by a truck, the information will still exist on the server.
2. Do NOT EVER store anything you actually want to keep in the trash. Not for reference, or anything else. That's just a mind-bogglingly bad idea.

And how did you do the upgrade, exactly?

Over the air? Using iTunes?

Do you have a backup of your phone from prior to the upgrade?

Did you get an error message during the upgrade at all? What did it say?

What do you mean by "email history"?

Who is your email provider?

How is the account set up? (IMAP, Exchange, POP3?)