[CentOS] getting old mail every 5 months or so

[Bob Hoffman]

Using centos 5.x, sendmail, as a server, downloading mail with 
thunderbird (although this happened with outlook too)

every 5 or 6 months I open my mail client (thunderbird) and one of my 
mail accounts decides to download 1,000 or so mails from my server. Old 
mail that I had already downloaded before.

I changed no settings and everything is the same as it was, but this 
happens enough, on all accounts, that it is just weird. It is like it 
gets to a certain size and then redownloads the same mails. And they are 
the same mails. I have had 'redownloads' of the same emails each time.

I look in the queue, nothing there...same with the mailboxes in the user 
folders, var/spool/mail....

these mails are being held somewhere by centos, but I cannot find 
them...and they seem to want to all be downloaded again and again every 
6 months..

what am I missing?

thanks