I neatly organize my emails into folders and subfolders. This works fine on my desktop computer and in the Web browser interface, but when I check my email on my phone or tablet, I don’t see all my emails. Why aren’t they synchronizing?
This is a common problem, but it’s not one for which we can recommend a guaranteed solution—there are tons of reasons why email doesn't work on devices, so you’ll need to do some digging. Obviously, your basic email settings are correct, or you wouldn't be getting any email at all. Each email provider supports many options in addition to the basic settings, however, so you may need to dig into the email options available on your device.
First of all, consider the “window” for which the device retrieves email. The default for your device may be small, like three days—if you have stored email older than that, it won’t be copied to the phone or tablet unless you change the setting that controls the email retention.
You’ll also find that some phones (such as the iPhone) and some email providers (like Apple) provide support for pushing email directly to the phone, rather than requiring you or the phone to fetch the mail. Using the Push option consumes more bandwidth and battery power than the Fetch option, but it does pre-load email onto the phone as soon as it arrives at the email server. Using the Fetch option allows you to fetch email on demand, or at regular intervals—this saves both bandwidth and battery life, and also allows you to (perhaps) feel less tethered to work, by retrieving email less often. You may be able to control these settings on a folder-by-folder basis, as well. Check these options to ensure you know how and when email should be synchronized with your device.
Finally, if you do modify settings that cause your device to suddenly pick up a large block of email content, you should consider doing this when you’re connected to a Wi-Fi network—retrieving lots of email over the cellular network could easily chew through your monthly allotment of data.