I connected my Pantheon e-mail client (included in Elementary Linux distro) to my IMAP e-mail service. I can then only see recently sent e-mails in the Sent folder, not e-mails I sent weeks ago. I can see ALL those old ones in my mobile phone client (iOS). Is and if so where is the config that changes this behavior?
Make sure to check all your folders. I have one folder called "Sent" and one called "Sent Items", that can be confusing.
You can also right click each folder and press "Refresh folder" to force it to download emails from the server. It doesn't give any feedback when you do this, so just wait a few seconds and see if emails start popping up.