macmail deletes my sent emails

Mac Mail only saves about 6 months of my sent emails. Where did the rest go? I'm trying to retrieve some messages I sent last summer, but they're gone. To make it worse, I decided to prevent this from happening again I should archive what's left in my outbox. so I selected all the messages and clicked "archive". The messages all dissappeared, and I have no idea where. I checked all my different mailboxes in the left hand side bar, but they're nowhere to be found.

I never want to delete ANY messages I send or receive. I use pop for this reason and I have "mail box behavior" set to "never delete anything". But somehow messages still keep dissappearing.