Emails keep disappearing?


I use Apple's Mail program with my POP3 and IMAP accounts. Random emails keep disappearing from time to time and it is causing me problems. I like to keep copies of all of my sent/trashed email in case I need to look at them later.

Does anyone know of a setting or something why this happens? Earlier this morning I had hundreds of sent emails in my Sent box now there's 8 which range in dates from the past 4 months. Is there anyway to retrieve the emails somehow?
In your 'Mail, Preferences...' window, with the 'Accounts' toolbar icon, an account, and the 'Mailbox Behaviors' tab selected - what is the 'Sent's 'Erase copies of sent messages when:' popup menu displaying? Is it anything but 'Never'? If so, I suggest you set it to 'Never'.

With the 'Advanced' tab selected, is the 'Remove copy from server after retriieving a message:' check box check marked?, If so, what is it's popup menu displaying? If not, it may be possible to recover all or some of the locally (those on your Mac) deleted e-Mail messages.
I currently have all of my settings under Mailbox Behaviors set to "never" delete/remove mail so it seems strange that they keep disappearing. I delete teh emails that are in my Junk box a couple times a day so I wonder if there's some kidn of bug in Mail that is also deleting the emails from my sent/trash as well (but not all of them)?