There are many reasons why emails might not reach your inbox.
A good first step is to try searching your inbox for all emails with the word "Edthena". This will help you find the email in question.
Below are ways to prevent the issue from happening again.
SPAM folder
If you haven't received an email from Edthena, check your spam folder in case the email got delivered there instead of your inbox. If so, select the confirmation message and click either "This is Not Spam" (AOL), "Not Junk" (Hotmail), "Not Spam" (Yahoo), "Not Spam" (Gmail) or the like, which will allow future messages to get through.
Gmail Inbox categories
(this also applies to Google Apps emails)
Gmail recently added automatic sorting of messages into three categorized inboxes: Primary, Social, and Promotional. If our message is in one of these other inbox categories, here's how you can train Gmail to sort it differently:
Gmail on Web

If you see a message in your inbox that you want in a different tab, all you have to do is drag and drop it into the other tab. Another way to do this is to right-click a message while viewing your inbox.
After you move a message to a different tab, a message above your inbox will ask if you want to undo that action or choose to always put messages from that sender in the tab you chose.
So, find a message from Edthena and drag it to your Primary tab to "train" Gmail.
Android
- Open a message from Edthena, or touch and hold one or more messages while viewing a conversation list.
- Touch the "Move to" icon. (If you don't see the icon, touch Menu, then Move to.)
- Choose the Primary category.
iOS
- Open a message in the Gmail app.
- Touch the more menu ▼ in the upper right corner.
- Touch Move...
- Select the Primary category.
Or, I haven't received the password reset email from Edthena