I know that the topic of this post seems to be obvious … however if you defined (or imported from a backup) a lot of email accounts during the configuration of your Windows 10 Mobile device (e.g. ten, as I did) you could be taken into this issue … as I was!!
In fact, once in Windows 8.1 you had one email app defined for each email you configured and you had a toolbar menù allowing you to move from one folder (e.g. Inbox emails) to another one (e.g. Sent emails).
Now, in Windows 10 Mobile (as it is in any Windows 10 versions) there is the Mail universal app [IT: Posta di Outlook] that allows you to handle all the configured email accounts. As usually happens in universal apps, there is a hamburger menù at the top left, from where all your emails are listed and, below that list, there is a Folder [IT: Cartelle] section where are listed all the folders related to the email account you selected in the upper list. Moreover, if in that selected email you have defined more folders (e.g. not only Inbox, Drafts, Sent items but also custom ones, for example one for each project you are following), may be you have to click on the last More [IT: Altro] item to see all the folders … may be even the Sent items one, being folders listed in alphabetic order.
If you are new to this app and you have many emails defined in your device, you can understand that you could not have scrolled what it is shows in the hamburger menù and so have not noticed the folder section, or may be not noticed the More item … and still wonder how to find Sent items!! 😉
Therefore, handling all your email accounts from one app it is nice, especially when you can see all the thread of exchanged emails for each communication, clicking on the last one received, … but you must habit to the new user interface 😉