Re: why is windows 10 mail not syncing?
Windows 10 on PC or on Mobile?
If on Windows 10 Mobile, you should go to settings -> manage accounts -> < your account name > -> Change mailbox sync settings.
then under the "Download new email" change it to "as item arrives".
If you want to get all your emails from before you set-up your account on your phone, then under the "Download email from" change the setting to 'any time'. Then wait for a while as the mail app will take some time before completely downloading. By "a while", I'm talking about a few hours to a few days. Depends on your internet speed, really. Rest assured, all your email should be there in the end.
If you're asking about the PC, then I'm really sorry as I'm not using W10 atm.