If you've forgotten an email account password, you can reset it from the "Email Accounts" section in the Admin Control Panel, as long as you have the "email accounts" administrative privilege.
First, find the email address you want to reset the password for and click the button in the "Reset Password" column.
Next, type in your new password. You'll need to make sure the bar below the "New account password" field turns green. This indicates the password is strong enough.
Lastly, click the blue "Reset" button.