If your account is IMAP then the server retention policies have precedent of any Thunderbird settings. What is your providers retention policy?

So - I called Shaw (my server) and they assured me there was nothing at their end responsible for the deletions. The problem started with a 2nd computer I purchased and installed Thunderbird on. The first computer had Thunderbird but didn't delete the emails ever. I looked at the 2nd computer and the account is listed as IMAP. On the 1st computer the account is listed as POP. I've deleted the new account and recreated it as POP. I really don't know what those labels mean but I'm hoping this will solve my issue. Thanks for your help.

re: I looked at the 2nd computer and the account is listed as IMAP. On the 1st computer the account is listed as POP.

It seems your POP account is downloading and deleting the emails off the server, so the IMAP account then will appear to have emails deleted because they do not exist on the server.

IMAP accounts can only show whatever is on server. All imap folders synchronise with server folders. In effect they are one and the same.

Pop downloads from the server inbox to the pop account Inbox in Thunderbird. Pop accounts by default download and delete off server no matter what email client you use. The server assumes you have downloaded and now have a copy on your computer, so it gets deleted off server. There is no synchronising between server and Pop Inbox. This means the server Inbox can look very different from the pop account Inbox. However, it is possible to force the emails to keep a copy on the server.

• Right click on pop mail account name in Folder Pane and select 'Settings'
• Select 'Server Settings'
• Select 'Leave messages on server'

Restart Thunderbird.