Messages disappear from the Thunderbird Inbox seemingly random intervals and are not found in Trash. My email provider is GoDaddy Microsoft 365. I was using POP3 until last spring when they announced no support for POP and the need to migrate to IMAP and I created a new identity with IMAP leaving the POP3 account there. Sometimes the POP3 will be downloaded with new messages too. All this started with the new version of Thunderbird.