Udate on 'not getting all my mail'

I have really appreciated the comments and info above from users.

I wrote a message about 10 days ago about receiving very little mail and no sent mail in the new Yahoo account.  I think now, after review, that since I was using Thunderbird as a front end and using a POP server, the ‘missing’ mail and sent mail was obviously in the TBird account on my computer. Which I was worried that it would just be a static, frozen data file.

Using the info I wrote down about the servers POP and smtp before the merger I wondered if I could merge the Cox-Yahoo new info into the existing account.  (Rather than having TBird create a NEW second account for Cox-Yahoo). 

I opened my existing TBird account and went into the account settings and put in the POP and SMTP info for Yahoo.  I had a few challenges since at first there was no OAuth2 in the choices in the drop down list.  After a little fiddling around and some restarts of TBird and one trip to Yahoo, suddenly that choice appeared in the list and I was done and it works fine.

So I wind up with all my mail and all my sent mail from before and all the new data from the Cox-Yahoo account in my 'new' original TBird account, my objective.  My idea of modifying the existing TBird account was reinforced by several of the messages above, and thank you for that added info.