need to ask some questions:
are you rooted? (probably not but need to ask)
has this happened since day one of owning the phone?
are you using the stock email app? gmail? some other 3rd party app?
how many email addresses?
Are you new to android and how the htc system works? or are you a previous android user?
if you are using the stock email app (not gmail), there is a tab up left that you can tap. opens up a drop down list and you can select which email address you want to check.
if you are in a particular email address, top right there is three vertical dots. top is folder. tap folder and this opens up another set of options on how to view your email.
all messages shows ALL messages of that particular email address.
so check those, respond to the questions, and report back to the class.