Problems Transitioning from Pop3 to IMAP in Outlook

For years, I have used Outlook with a Pop3 configuration. I would like to transition to IMAP, but I'm encountering a problem in Outlook that's making this a virtual impossibility.

Over the years, I have created numerous folders in my Pop3 account. Obviously, it's easy to recreate those folders in IMAP and copy/paste the emails into their new home. But after completing this step and waiting for sync to finish, I logged on to Gmail and noticed that several folders were missing messages. Upon further investigation, I discovered that the missing messages all had one thing in common: they were messages I had sent. For example, I might have sent an important message to a vendor in Pop3 and subsequently copied that message from my sent folder into a folder called "authorizations" for future reference. In these cases (and in every case where I was the sender of the message), the copies of sent messages are not displayed on the Gmail side. 

What's mysterious is that the folders look perfectly complete on the Outlook side. It's as if Outlook is simply failing to upload copies of sent messages to the server, but it retains them on the desktop client. This, of course, defeats the purpose of IMAP. As a result, when I set up Outlook on a different device, the folders on that device are a perfect reflection of the Gmail side (i.e. messages are missing). The only way to access the missing messages is to use Outlook on my original device.