That's not nearly enough information.

Are you using POP or IMAP? What are the account settings? If IMAP, do other devices also access the same account?

I found this... Server Type: IMAP Mail Server ... I don't know anything about these settings... Port: 993 Server Name: mail.usgo.net Connection Security: SSL/TLS

Most folk making post of this nature have IMAP accounts and are not aware their provider has retention setting that are ovriding what is set in Thunderbird. For instance many automatically delete mail from the trsh after a set period. Others also automatically archive mail after a set time or some even delete it.

IMAP is not as simple as making some setting in a mail client as was the case with POP mail. IMAP relies on settings in the client and on the server, so you have to understand both to get it right. I suggest you contact USGO and ask them what if any archive and email retention and auto aging policies or settings they may have.