I try to put an email in 1 of my mailboxes but it disappears and I cannot find it

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.

Boot from the HD simply means hold Shift key on start up.

Mac's Mail App is not as modern as iOS's Mail app, Many eMail providers now require a separate App Password different from your eMail password…

Yahoo has gone to this too...

https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html

Yahoo cure, thanks to tekkaman65535…

yahoo mail failed to authenticate - Apple Community

yahoo mail failed to authenticate - Apple Community

Yahoo App password…

Generate and manage third-party app passwords

Some older, third-party email apps (that do not use our Yahoo branded sign in page) require you to enter a single password for login credentials. To access your Yahoo Mail account on these apps, you'll need to generate and use an app password. An app password is a long, randomly generated code that gives a non-Yahoo app permission to access your Yahoo account. You’ll only need to provide this code once to sign in to your third-party email app.

https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html

Most likely biggest factor is using Other in Mail setup, then using a Browser that doesn’t have Yahoo PW stored init to get the App PW.

Changed Yahoo password, yahoo stopped sen… - Apple Community