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. The password will need to contain at least 8 characters with a letter and a number. You'll also need the progress bar to turn green, which indicates the password is strong enough.
To double-check that the password has been entered error-free, click on the gray "Reveal" link next to the password field to view what you've entered.
Lastly, click the blue "Reset" button.